Well, here are some questions you can consider.

1. How strong are your character’s abilities? How far into the past/ future can she see? Is one direction (forward or backward) stronger than the other? This might add a touch of realism.
2. Is your character instantly good with her psychic abilities, or does she need to develop them?
3. Does she use them in everyday life, or just for saving the world? (And I don’t just mean for big things, like cheating on a test. I mean looking into the future to see how her second date will go, looking into the past to see what her coworkers said about her, etc.)
4. How do they work, exactly?
5. What happens if she overuses them? Does she get headaches? Do her visions become fuzzy? Do her predictions become less accurate?

Not all of them need to be answered, per se, but answering those questions could help create a sense of realism. By simply knowing her powers as well as she does, you’ll make your story that much more realistic.

I have a character who is infected with “demon spawn,” and when he gets overly emotional he turns into a demon. The upside of this is that he is super strong, fast, basically enhanced physical and mental abilities, and he is nearly indestructible. But the downside is that he can’t control himself, so he usually goes on long killing sprees while as the demon. That’s one way I strike a balance.

Zigackly.

I don’t personally have this problem so much any more, but when I was 13 I had an Evil™ warlord character who was (according to my surviving notes) schizophrenic, manic-depressive, sociopathic, had delusions of grandeur, was epileptic, had multiuple personalities, AND had a homosexual Oedipus complex. All at once.

I….I don’t know whether to laugh, cry or applaud. :P

My witches have to give up energy/strength for magic powers. Basically this means that the really strong ones can summon extremely powerful flames, storms etc., but if you grab their wrist and apply enough pressure, it will snap without much effort. Also it’s energy intensive, so they have to guess whether they have enough energy to use a certain type of magic, or else they’ll probably die. As a little side note, I imagine that they are constantly munching on high-energy foods in order to store up energy.

So yeah, I am an advocate of balancing power.

There are two types of magic users in my universe: those who are born with their powers and those who buy them. I haven’t really thought too much about the latter—it’s a black market trade, and very dangerous—but I’ll get to that soon.
The people who have magic from the start tend to be physically impaired in some way; Cecile (the psychic) was originally going to be blind, but I realized I wouldn’t be able to pull that off and instead gave her a bad left leg.

Most of my important characters have spiritual power that’s basically magic with some Eastern concepts pasted over it, although it functions more like the Force than anything. I try to keep it reasonably mystical even though there are scholars in-universe that study it scientifically. I wouldn’t get too carried away with trying to explain your supers via technobabble, since it invites readers to poke holes in your logic instead of shrugging and accepting it.

I don’t worry that much about forcing “balance” onto it, because that can easily make it feel like an authorial contrivance. If you were to put it into real talents, look at a freakishly talent person like, say, Usain Bolt. What is the trade-off for him being the fastest man alive? There isn’t one. He’s just really fast.

I don’t think it’s that big a deal for my story in specific, since, like I said, most of the important characters have it, and most time it just means higher than normal speed/strength and rapid healing.

For purposes of narrative tension or whatever, the female lead can heal others and use telekinesis, both of which are uncommon, but she’s so crappy at pacing herself that she runs out of gas quickly. That comes more from her personality than me wanting to give her powers a flaw, though. The male lead technically has it too, but his is basically inert, so the only thing he gets from it is being invisible to people who can sense it.

There are some other unrelated powers too, for people without the spiritual gift, like a character who is almost always in a state of semi-sleep, and gets stronger the closer he gets to complete sleep.