get really good at using the right stick to finish tricks right before landing, and once youre good with it itll help add spins to your tricks (for example, a lot of the time to do a rodeo 7 or something you need to stop pushing the left stick and finish the trick with the right stick)
dubs and trips you just need to get used to spinning/flipping right near something that will trick it into flipping faster. since you have 3, either download some people's parks (go to google and enter site:newschoolers.com + skate 3 parks or something, there have been some solid threads with people posting their gamertags/parks) or make your own that have big jumps with pieces placed just under your flight path to dub and trip off of. you'll usually have to pop mad early too (this is true most of the time even if youre not doing dubs etc anyway)
that and just do it a lot and youll get way better obviously from practice. let me know if you have any more questions or whatever, i've played waay too much skate, i used to just throw it on any time i was doing school work and take a couple hot laps every so often