Well, buying deco will do it. Buying chairs and tables will do it. Buying appliances will do it. Any time you spend coins, you gain XP. Tipping your neighbors doesn't get you XP, but collecting tips your neighbors have left you will.
I still think the best way to pump up XP is Jasmine tea. Each bake is 1 hour and 84 XP, on a dozen ovens (lvl 46, unless you've opened slots) that's over 1,000 XP an hour.
(I
highly recommend spending gems to open oven slots, too!)
And neighbors. You will need up to 61 neighbors if you're buying expansions with coins. If you're expanding on gems you just need a boat load of gems.
The
"Welcome to Bakery Story" thread has some great information! Here's the
Expansion Requirements.