Last year, I complained about my iPhone 3GS home button losing sensitivity and becoming increasingly difficult to press. A year later, my iPhone 4 home button has the same issue, and worse.
A faulty home button could be the result of several things. It could have water damage, it could have dust clogged beneath it, or it could be intentionally planned by Apple to become obsolete.
If your home button is hard to press, your best bet is to take it to the Apple Store and see if they can replace your phone at the Genius Bar under warranty.
But if, like me, you live far away from the nearest Apple Store, or if your warranty is void for whatever reason, then I advise you to try this unconventional, but somewhat-effective fix for a faulty iPhone home button: suck the dust out of your iPhone charging port with a vacuum cleaner.
I read about vacuuming your iPhone in an Apple Support forum and was skeptical, but I tried it and, although it did not solve the problem completely, it worked to an extent. Pressing my home button no longer stresses my finger as much as it used to after trying the vacuum fix.
Just connect the hose attachment to your vacuum cleaner, turn on the vacuum and put the hose up to the charging port at the bottom of your iPhone.
You may want to use a pair of tweezers or a needle to take out dust particles as they are pulled into the charging port.
Do this a few times, and if you are lucky like me, then your home button will be at least slightly easier to press.