Through some of my time racing in this game, i have found out many things about the mechanics of boosting. This is what i know so far, and i hope they can help you to be an more efficient driver!

Starting Boost: This does not increase your top speed, but does give you some speed right off the bat. You can activate it easily by accelerating right when the race starts. Still if you press too early, you will get a penalty!

Drift Boost: When drifting, you can gain a speed boost if you let go depending on how long you have drifted for. Yellow boost comes out very fast, accelerates you and raises your top speed. It is very powerful, and if you aren't in a yellow boost state, you should. Red boost comes out much slower, but does last longer and has a higher top speed. Be sure to use it fully though, as yellow boosts will override it.

Nitro: Nitro temporarily increases your speed cap, but it will not accelerate you faster! This means it is ineffective to use nitro if you are not already at top speed. When using nitro, there is a minimum time it will be consumed for. Dugglez: By pressing the nitro key slightly before you pick up the nitro you can use a small bit of it and keep 2 more charges.

Zipper: Instantly propels you at top speed, i leave that to your creativity!

Conclusion: The best thing about all these boosts is that they will stack on top of each-other, be sure to use each effectively and have fun!

That is my guide for now. It is very possible in the future that the boosting mechanics will be changed, as STK is not entirely complete.