You've seen it before. Ensign John Doe singlehandedly mudwrestled the Borg Queen to the ground, having resisted assimilation and then proceeds to blast his way through the Borg defenses, flies a shuttle in and out of a black hole, and saves the universe.
Many people are in some minor way guilty of this disease. Their characters are perfect. They're the characters you'd love to be.
There is a pitfall to avoiding these characters, of course. The Victim Syndrome. No matter what, these characters are the ones that can't do anything write. Ensign Jane Doe is a counsellor who's patients commit suicide after the first therapy. She is the reason that the ship is now the USS Heart of Gold-D
How do we avoid these rules? Well, here are my opinions.
A) Find a weakness for every strength.
Perhaps your character is an excellent counsellor. But he is an atrocious fighter, and can't even aim a phaser right. And even down in one set area, narrow it down. Perhaps she is great with grenade launchers and phaser cannons, but when it comes to sniping or flying, she's better off just ducking.
B) Not always do your strengths work.
Alright, so your character is a perfect marksman. But even the perfect marksman misses sometimes - perhaps this will lead to an interesting character developement later?
C) Strengths/weaknesses are not always practical skills.
Perhaps your character is a good marksman, but has trouble getting himself to pull the trigger. Perhaps he is a skilled doctor, but is grouchy and is no comfort to his patients.