Try to work on keeping the car balanced and keeping your inputs as smooth and progressive as possible.
If you balance the car correctly, the car just seems to 'float' over bumps and curbs. Doing so means you can carry a higher apex speed by getting off the brakes sooner and back onto the power earlier, while having to apply less lock to get through the corner. Try to keep the car 'flat'. A heavily-loaded wheel does not tolerate bumps and curbs well. Turn into the corner smoothly and decisively to set yourself up properly, then balance the dynamic mass of the car through the apex and exit by subtly manipulating the steering and throttle.
The inside wheels are much less loaded and under less stress than the outside, so they're generally much more tolerant of riding the curbs. Curbs basically extend the useful width of the track, so they should be used whenever it is beneficial to do so. Generally, the more of the track you use and the less lock you apply, the faster you'll be.
I often almost get rammed up the back (well, sometimes not almost) on the entry into a corner by people who are significantly slower than me. It's definitely too easy to be too aggressive into a corner while thinking you're the 'bestest driver evar' only to watch the car you just road up on go flying up the road on exit. If you're continuously powersliding in on entry or missing your apex, you're trying to enter the corner too fast. Try to find the sweet spot where you can keep your tires at a constant murmer both front/rear throughout the entire corner. Learning how to create lines/braking points comes with experience, but it definitely helps to watch a truly fast driver go at it. But keep in mind that if you want to maintain their kind of entrance speed, you have to duplicate each one of their control inputs with precise levels and timing. So it's better to aim a few percent lower in speed.
One thing to aim for is trying to find the exit lines (not apex speeds exactly) on the corners with straights after them where you can pretty much whip your car's throttle straight open. You may find yourself taking a much different approach on entry in an attempt to maintain straighter lines on exit.
If you want to get more technical, download f1perfview and output raf files from a 'smooth' wr lap and run your own laps for comparison. You'll probably be surprised at how aggressive the wr lap is with throttle, plus it really points out differences in your lines.
And as to the actual topic...
It's often times useful to use curbing, but often times it just upsets your car. Generally, the higher the load you have to put on your car while going over the curb, the less you'll want to use it. But of course that varies with the geometry of the curb itself.
man, I just tried some guys setup who got 1:13:10 on it and I just find it so difficult to drive with that setup! I spin out so easily and I have no clue how to accelerate when exiting a corner.
Ohhhh yes that's normal. Their setups are usually on the edge. You probably want to take advantage of that extra oversteer when you enter in the corner by entering in just a little bit faster without understeering. Dive in as far as you can in the corner and then add more throttle once you close in to the apex (quickly but smoothly).
hrm, some guy online who can get 1:13's consistently suggested that I turn into the apex while braking into the corner.
Problem is that I'm using mouse and once I try to turn, my wheels sort of lock up and straighten out and it starts to get really difficult to control the car where as just braking ina strait line, letting go fo the brakes and turning.
I always get the braking done in a straight line and then turn in. Why would you turn in as you are braking... you just understeer . He might be talking about trail braking.
I guess trail braking always helps... I leave the brakes on for a fairly long time tbh. If you brake later, you obviously need to spend the same amount of time stopping... So therefore you'd have to brake while corning. Basically all trail braking is, is as you steer into a corner, slowly let of the brake untill you're at or a bit before the apex, then accelerate off the corner as fast as you can. The faster you can get on full throttle, the faster your exit speed will be