This thread is setup for people that have found tips and tricks in the game, or know of them, without using codes. Hopefully we can keep the topic like this for as long as possible. I will start with a good money trick I found out about on the net that I used to go from about $1500 to $24.6 mil in about an hour (real time, not, gta time):

go to the horse betting place, put all of your money on the 12-1 horse (sometimes it is 11-1). If you lose, simply reload, and try again. If you win, go save the game. Keep doing this, and everytime you win, you will get 11 to 12 times the amount of money you have and that will be added to the money you already have.

The last mission I got through was the ddr-type mission, and I am sitting on 24.6 mil with no codes entered yet.

Health trick:
this is something that was used in vice city and I am pretty sure it was used in 3: if you are low on health, go to a safe house, go to your save point. I always save at the save point, so I can't vouch whether this works when you don't save, but it will always fill your health.