When your character levels up, you have to go through a process where depending on their class and level they will get certain statistical bonuses that are randomized. By saving after your character attains enough experience points to level up but before you actually level up the character, you can manipulate your statistical bonuses, resetting the game until you get the statistical bonuses you desire.
For instance, in terms of hit points, which is based on a character's constitution, you can raise your character's hit points by waiting for huge constitution bonuses, resetting your game until that bonus is attained. This is a really useful trick to make hugely powerful parties. It just requires some patience.
Try the following if you want to earn a seemingly-infinite amount of experience points. Head to the Spider Spike Hollow, and battle your way through the hordes of Trolls, Goblins, and Bugbears. Then, head to the beginning of the cave and battle in another direction, towards the location of the Giant Spiders. In the cave where those spiders dwell, you can head even deeper into the cave. However, go back the way you came. You will return to the Game Map, but your experience (and gained levels) will be retained.
If you want infinite amounts of gold (and have patience), try this technique. Buy and equip a Buckler, and head to the Eastings Merchant, who has an incredible array of powerful gear. With the Buckler still equipped, opt to sell it to the merchant. You'll get 7 gold for it, but it'll still be equipped on your character. You can then do this over and over again for infinite gold pieces, but it'll take a while to build a substantial amount up only seven pieces at a time.