Tricks occur randomly whenever you get into the air with a bike or quad. They appear to occur more when you jump off a ramp type thing, but thats just my experiance.
A good way to grind this is to do all your livery/vista hunting on a good offroad bike - you should grind out plenty of tricks while hunting all the collectables. Or you can just find a jump which reliably gives you them and then do it over and over again.
It is also worth bearing in mind that many bike races will involve trick giving jumps, so you will get some progress just by playing normally.