Okay, so I had this problem for a while.

I usually start any song rather relaxed, but as the songs goes on, fingers get tense and tense to a point where I cannot play accurately and is so tense that I cannot play fast parts (tense fingers makes me slow).

I've searched around for tips and tricks to do the magic, but I have one thing I don't understand.

I know that relaxing, posturing, and using only minimum force, just enough to fret a note is enough. But when you bend a string, or do vibratos, I will need some force in my fingers so that the strings won't slip out. Using minimum but enough force to bend a note or vibrato seems like making my hands tense enough that I get sore palm and fingers.

Fast solos with vibrato and a lot of string bending will obviously not work because if I relax my fingers enough to play fast, then I won't be able to bend a string, and I will not be able to use wide vibrato neither. If I use enough force to bend a string and do wide vibrators, then my fingers are too tense to play fast.

I don't really understand this "relaxing" thing. How do I properly relax my fingers?
I had that same problem for a while. For the most part, I still do unless i get real warmed up properly. Try some finger stretches, shake ur hands good that does something scientific to your joints or acid or something, and play a lot real fast and get a good burn going in ur hand then relax for a while and go back to it. Those things usually work for me
Quote by PSimonR
Meditation, Tai Chi, Qi Gong?
Practice slowly with minimum movement.

I don't know if this will help but I have a terrible habit of strangling the neck and pressing down on strings to hard. Not only will this slow you down and prevent faster clean hand movements it also will tire your hand out, which means less practice/playing time. To help me stop this my teacher has me apply pressure to a string and continue to hit the note using less and less pressure till I find the point where its hitting clean, without killing my hands, strings, tuning and frets. I repeat this all the way up and down the fret board on each string to fret twelve. Its helped me tremendously. Good luck.
Okay guys, thanks for the tips. Actually I'm starting to feel improvements already.
It's the same old advice ... when you practise, for at least some of it, play really, REALLY slowly. When you lift you finger off the fret, tell yourself to relax the finger off the fret, rather than to pull the finger off the fret. Experiment with only having one finger fretting at any one time (for soloing), but you have to get muting happening big time with the picking hand.

Don't expect to use the same strength (or lack of it) all the time. Legato can be played as above.

Bends and vibrato can't. For bending you need to learn how to lock your fingers and use a combination of squeezing your thumb towards the bending fingers (where possible, try and squeeze two fingers into the fret your bending at). For vibrato, you need to bring into play your forearm, using your first finger as a pivot against the neck.