You need: *A Power Seed
*Full field upgrades and a heap of dung or droppings (you can do it without these but it takes FOREVER!)
The cheat is simple but it get's the job done. You simply plant some power seeds (worth 140 each), use dung or droppings (easliy found in many areas in the Desert Zone) to fertilze the fields, come back after you've completed a misson, harvest the seeds, replant and repeat. You sell the seeds when you've got 100+ Power Seeds which gives you about 100,000 whatever currency you use in Monster Hunter.