There's a lot of ways, most of which happen after you've hit 20. Normal monsters give you a miniscule amount. The best way to grind it is god kills (the hard monsters in the 'farlands' I think it was called) but there's ways to multiply it. Off the top of my head I can remember something like 10% of your kills being god kills gives you a bonus, never using potions, accuracy, even weird ones like never do any damage (just sponge all your xp and fame off others). You've probably seen equipment with fame bonuses as well. Check the wiki, there's a list somewhere. All in all it's very slow to get and requires you to be able to survive in the harder areas, you really need to play in big groups to have any chance and learn all the attack patterns of the god monsters. If you're naturally good at bullet hell style games it might be quite a good challenge but to me it just felt like a grind in the end.