The key to barre chords, and chords in general, is that you don't need to clamp down hard on the strings, you want to get maximum output out of minimum effort.
If you find yourself holding the strings down very hard, then some will go sharp, causing your chord to go out of tune.
This happens a lot with barre chords as people believe they have to have their first (or whichever finger you're barreing with) fretted hardly.
Try and keep the finger as close to the middle of the fret as possible.
Try and straighten the finger you're barring with as much as possible, this will ensure your barre is consistent throughout all of the strings you're fretting. To straighten your finger drop your thumb down the back of the neck slightly so it's 3/4 'up' the back of the neck, doing this will force your wrist to move around in an arc and should create a straighter line from your wrist to your fingertips, this'll seem awkward at first but it's the key to fretting
any notes with clarity and precision (take a look at how some exprienced classical guitarists position their fretting hand).
If necessary, don't be afraid to 'roll' your finger onto the side
slightly, although you want to avoid this if possible.
Barre chords suck.
EDIT: BTW F Major barred is especially difficult (presuming you're playing it in the first position: 1st fret) as here your strings are at their highest on most guitars and also retain the most tension as they're nearest the nut. You might find it easier to try fretting an AMajor barre chord (move up your chord to the 5th fret) or a BMajor (7th fret) first as here you're around the 2/5ths/half way point and the strings should be easier to fret. Once you get this down try moving your chord further down the neck.
Hence the long post 
, they do get easier, i was practicing and i sucked, then the next day i was a lot better, then left it a bit and went back to my picking, then back to barre chords and i can actually do them now 
