squeeky brakes!

i have changed my brakes discs and pads about 3/4months ago and done a good job, but recently has started to get squeeky brakes not right away after about 20mins of driving, the noise sounds like they need changing again like the noise you get when they do need changing if that makes sense. i wouldn't say the parts i used were cheap but they certainly weren't expensive, any ideas??

my car 1.5 almera s, no abs, 3 door!

no never done that, do you mean using copper grease?

no i never done that, im hopefully going to do it 2mz and that should sort it, thanks for the help, i'll leave comment back let you know how i got on.

i gave the discs a wipe with a rag and before putting the wheel back on sprayed the disc with brake spray cleaner then wheel back on, but i did notice and faster then 70mph i get a vibration in the steering wheel would that have anything to do with the squeaky brakes?

ive done the job now and appears to be better, the vibration has been reduced and ive heard no squeaking so far, fingers crossed. ive still got a small amount of vibration at 70mph but im putting that down to the fact i do need the tracking and balancing doing and im bound to get a small vibration at speed. however i did snap a bolt when i put the caliper back on, the small one thankfully the one that goes into the small pin/bolt thing luckily its removable so i bought a new one and there are a nightmare to get hold of, incase it happens to any1 else i got a set from http://www.brakepartssuperstore.org.uk for £22 inc delivery...