Definitely work on places like Shy Guy Beach. Also, at your level, it would be better to use Toad, since he is easier to get decent times with, and you can corner tighter with him.  

Unlike Hahaae, I strongly recommend you stick with Bowser, following the same advice I was given when I first joined: Bowser is better anyways, and the longer you wait to switch the harder it is.
Also, besides the basic "learn to corner more tightly", I think it'd really be worth learning to waterslide, because it's a technique that's useful on a large number of tracks (including some of your weaker ones like SGB) and the basics are fairly easy to learn (compared to other strats).
If you're willing, there are some more "high risk, high reward" type of strats, that are much harder to pull off, but if you do you are guaranteed to get massive cuts. These strats include CCI gap jump, SG advanced strats, RR advanced strats for example. If these interest you, I'm more than willing to give you pointers on any you want to try.

Damn writing messages on the 3DS browser takes so long...
-Christophe.

P.S.: I was also very puzzled when I saw this as the last thread posted in the MKSC section, thought someone might have renamed the Official Times Update Thread.

I would suggest that you try to master water sliding. It's not hard at all and it's a great help for every track that has water or snow in it.

And at your level, just playing a lot makes you tons better. I don't expect you to have a lot of trouble beating your entire current timesheet. First try to make the big time cuts without too much trouble, then, when it starts to take you longer to PR, go for new strats.