After killing all the monsters and NPCs in a mission, try to exit through a different door than the one that triggers the end of the mission. This ends the mission without canceling the experience points. When you go back to the campaign map, the mission will still be active and repeatable. Never level up more than the required level for the mission or you will not receive the experience for the specific level monsters. This is based on the fact that wizard made the game exactly off the core rulebooks of AD&D, allowing you to play the same level repeatedly until you have enough experience to get to level 20.
When your character levels up, you must go through a process where, depending on their class and level, they will obtain specific statistical bonuses that are randomized. By saving after your character acquires 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 want 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 large constitution bonuses, resetting your game until that bonus is acquired. This is a really useful trick to make incredibly powerful parties, but it does require some patien
To a seemingly-unlimited amount of experience points, go to the Spider Spike Hollow and battle your way through the hordes of Trolls, Goblins, and Bugbears. Afterwards, head to the start of the cave and battle in another direction, towards the location of the Giant Spiders. In the cave where those spiders dwell, you can go even deeper into the cave. However, go back the way you came and you will return to the Game Map, but your experience (and earned levels) will be retained.
If you want unlimited amounts of gold (and have patience), purchase and equip a Buckler, then head to the Eastings Merchant, who has an impressive arsenal 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 repeat this over and over again for infinite gold pieces, but it does require patience to build a substantial amount up only seven pieces at a time.