This is what works best for me Go to Settings->Applications->Manage->Running - Click Market - Then Clear Cache (NOT CLEAR DATA!!!!) - Then Force Stop - Click Google Services Framework - Click Clear Data - Click Force Stop
Then reboot your phone.
If for some odd reason this still doesn't fix it what you can do is remove you current google account by using root explorer and going to /data/system and rename accounts.db to accounts.db.bak, this will remove you current google account. simply reboot the phone after doing this and add your google account again upon opening up the market app.