Since lvl 6, now I'm lvl 8, I've been attacking creatures at my lvl or 1-2 higher and my bar doesn't move. My buddy started a new guy when I was lvl 6 and before I made it to lvl 8 he made it to lvl 10.
The best way to get xP is just do quest. I went from level 20 to level 21 in just under an hour. Just make sure u always start a quest before walking by and forgetting to ensure u won't run out.
It's possible that you are getting XP and you just can't tell. For creature kills, the amount of XP earned may not be enough to obviously move the bar.