I was kind of psychic as a kid, but after growing up with extremely skeptical parents and having one too many weird things happen, my abilities shut down in my early 20's. For the past few years, I have been working to get them back, and am having a fair amount of success.  I have literally gone from being mildly psychic to being good enough that people pay me for readings in less than 2 years.  Here are some things I've learned that have helped me:

Tips for Developing Psychic Ability in General

1. Meditate.  Meditating helps your mind learn how to stop and let the information come in for a change.  If you don’t learn to do this, then your mind ends up so busy with it’s own thoughts that it can’t notice anything that is coming in from the universe.  Don’t get frustrated if you’ve tried meditating unsuccessfully in the past.  You don’t have to concentrate of your navel for hours at a time for it to be effective.  Explore different ways to meditate if the traditional “concentrate on your breathing” or guided meditations variety don’t work for you.  You can do active meditations like counting from one ten continuously while walking.  The meditation that really helped me was the Three Minute Meditation by Peter Russell http://www.peterrussell.com/TV/3Min.php   He keeps changing the focus of what you are concentrating on and reins your mind back in when it wanders.   And yes, even meditating for a few minutes at a time can be helpful.

2. Learn to trust your abilities even with the mistakes. University studies have rated even the best psychics as being right only 85% of the time. But being right 85% of the time is nothing to sneeze at! You may also find that the information coming in is correct, but how you interpret it is a little off. There's always a learning curve. That's fine. The more you practice, the better you'll get, but it does take time.

Avoid skeptics until you’ve developed a fair amount of confidence in your abilities.  Talking to someone who blows your abilities off as coincidence, self-delusion, or the work of the devil will undermine your self confidence faster than anything.

3. Don't try to force it. You'll block yourself if you try too hard. Some of the clearest communications I get come through when I'm not even trying at all.  If you are getting jammed up, go do something you enjoy for awhile before trying again.

4. Join a psychic development group.  You can take a psychic development class, talk to the folks at the nearest Spiritualist Church, or join a group online.  The important thing is to be around likeminded people.  There are a few reasons for this.  It acts as a support group for you while you are battling your doubts about your abilities.  It gives you a sense of all the different ways people can perceive things psychically and all the variety of psychic gifts people have.  Also, being around other people with abilities tends to boost them for everyone.

Go by your gut feeling when choosing a group.  There are some out there with a very competitive atmosphere or where the instructor is using it to feed his or her own ego.  Thankfully, there are also a lot where the atmosphere is friendly and supportive.  That is what will help you the most.

5. Practice, practice, practice.  Again, being in a psychic development group is good for this.  You can take psychic tests like can be easily found on the internet.  For some people, though, that puts them in test anxiety mode, which interferes with their ability to relax and let the information flow.  Others see it as just a game and have no problem.  If you are the test anxiety type, there are less competitive games to play, too.

One game I like to do is to go to museums and antique shops and mentally reach out to the objects to see what I can pick up from them.  If you got to an art museum, for example, you can try to pick up on what the artist was feeling while working on a portrait, or what the subject’s life and personality were like.

6. Learn as much as you can through books and radio programs.  I listen to several online paranormal radio programs and have learned a lot that way.  Again, this gives you a better sense of all the different ways psychics can pick up on things. That helps keep you from thinking “well, I’m not picking up on it the same way as the famous psychic does, therefore I must be wrong.”  You can also get a lot of tips on how to improve your psychic abilities, too.

Hay House Radio www.hayhouseradio.com has some excellent weekly programs by Collette Baron-Reid, Sonia Choquette, and John Holland that I have found particularly helpful.  Darkness on the Edge of town www.darknessradio.com and Haunted Voices Radio www.hauntedvoicesradio.com also have archives full of interviews that you may find helpful.  I also recommend Chris Fleming’s Spirit Talk www.spirittalk.planetparanormal.com and commentaries on his program Dead Famous www.deadfamous.planetparanormal.com

Reading about the lives of other psychics is helpful.  One misconception people have about psychics and mediums is that they were born fully psychic and have always had ready access to their abilities.  The more you read about other psychics, the more you will find that although they may have had some abilities when young, most of them also had to work hard at developing their abilities to get them to the level they are at today.
 

Tips Specifically for Mediumship

1. Mediumship for most people isn’t like you see in the movies and on tv.  Mediums don’t necessarily see and hear the spirits the same as if they were here in physical form.  Most mediums perceive them in their mind, like a daydream.  You may get fleeting mental images of them or hear thoughts that sound like your own mind answering the questions you ask.  As long as you are getting the message, it doesn’t matter.

2. How do you know if that other voice in your head is really a spirit or just your imagination?  You can look for clues in the speech patterns that differentiate it from your own.  In my case, I realized that a voice in my head that claimed to be a Civil War era ghost never used contractions in his speech and occasionally threw in some archaic terminology.  Again, as you get better at it, you may find that the voices themselves become more distinct instead of sounding like your own thoughts.  Also don't worry if when you do get better at hearing them in your head verbally, they sound like your own thoughts. Yeah, some mediums do hear them in their own distinct voices, but even some highly gifted ones like John Holland don't.

3. Don't worry that the dead communicate with you symbolically. That's really common. They do their best to communicate with us, but that's often all they can manage. A ghost I deal with a lot likes to communicate through music. Others will radiate an emotion. As long as they get the message across, that's the important thing. A lot of psychics find that over time, they develop a symbol dictionary where they can reliably know that if they see a cow, for example, that it means the dead person was a farmer.

Protection

Don't forget to always use some form of protection when working with the spirit world.  There are many techniques for this.  The most common form is shielding.  What I use is basically mentally filling myself with a bright white light, pushing out any negativity in the process.  Then once I am filled with light, I visualize putting on a flexible suit of armor made of purple light.  This suit of armor acts as a filter that I can adjust to let in as much or as little psychic information as I want.  This allows me to completely shut out psychic energy when I need to, or to only allow in certain kinds of information, such as communication from a single spirit.  The armor is always set to block out negative energy completely.

Another concept that I find helpful is cloaking.  Unlike shielding, this does not block negative energy but instead keeps it from being attracted to you in the first place.  After shielding, I also imagine covering myself with a beautiful purple cloak.  This cloak filters outgoing energy such that positive spirits can see me, but I am invisible to the negative ones.

Respect

Spirits are people, too.  Just because they are in a different format doesn’t mean that you should treat them any less respectfully than you would a person who is still in physical form.  Introduce yourself to them first and explain why you are contacting them before expecting them to start answering a bunch of questions.  You will have better luck talking to them conversationally than acting like you are interrogating them.  Be mindful of the time period they are from and try to adhere to the standards of courtesy that were commonplace then.  Examples would be to refer to them as Mr. or Mrs. instead of addressing them by their first name, and not using profanity.

Good luck!