Be sure to kill your enemies in combat, not using spells; Ultima V doesn't award experience for magic kills. And kill them, too; only the actual giver of the death blow gets experience.
My preferred hunting ground, incidentally, is the bridge just east of Britain - sit there for a while and hammer trolls in to the ground. That's generally where I go to start out and gather the necessary stuff to launch my party into the wilderness.
- Note that top level is level eight, with 240 hp. In Vas Grav Corp may well be your best friend, especially when challenging mongbats show up in numbers enough to swarm over a small eastern European satellite nation.
- If you're feeling especially cheeky, go to Blackthorn's Keep, get tagged by the guards, and refuse to go quietly; sure, they have crossbows, but if you have heavy armour and some good support magic (Quickness, Protection, Invisibility and the like) you should be able to pull out a few high-experience victories. Blackthorn also has some other nasties about that are quite willing to fight you; just don't challenge the Lord himself (he's indestructable), and bail out immediately if any of the Shadowlords decide to pop in for a visit.
- The Magic Axe is your friend. Heavy damage, and you can throw it across the map. The Halbard or Morning Star are also handy - you can hit over obstacles with them, which can be very useful in dungeon rooms.
- Camping can get you ambushed; that may not be an entirely bad thing. (using the camp to gain experience is asking to run out of food, though)
- Lower levels of the dungeons have room with some powerful bad guys in them. Gargoyles, daemons and dragons are worth large bunches of experience - especially the divisible gargoyles. Be careful about fighting daemons; their ability to possess party members turns an easy win into a party slaughter, and deprives you of much-needed points. Don't challenge them too quickly unless you've got some way to negate magic.
- um... you do have a way to negate magic, right? (you should, if you're talking about taking on Doom)
- Finally, exploring the Underworld can also lead to many riches and much experience. And it can get you really, really dead.