pretty simple really. When going through act 1 here's what you do.
Get to leoric's mansion quest.
Hit the checkpoint right outside
go in mansion
kill the SWARM of mobs that are always there.
Leave game
Resume game
Repeat.
I am leveling my wizard who was 15 when I started this a bit ago. she is now 18. I get 4-6k XP per run on normal. would be a lot more on hell. but for low level leveling it is working great.
Fast Xp, 4-6k for about 45seconds work.
Note: AoE makes this a lot faster.