Buying equipment
Sell off all your lights as soon as you get in the shop. You won't need them until stage 6 (and even then they're not important until stage 7), so selling them gives you a bit of extra money. If you don't use your brakes — releasing the gas pedal usually slows you down enough — you can sell them off too. If you do this, you will probably have enough money to max out engine, tires, and shocks, and still have enough money for a few nitros.
Don't forget that you are on the clock while you're in the shop. Don't waste time buying nitros and such that you don't need.
In-game strategy
This guide assumes you are playing on Expert mode.
General driving tips
The brakes (
) are awkward to use if you use 
to drive. (You can also hold down 
to drive, but it fires a nitro when you press it.) It may be better to watch where you're going and just let go of the B button if you need to slow down.
Note that, since maxing out your essential equipment is so easy, you can essentially ignore item pickups if you're at all decent about avoiding buggies and spectators. In particular, there's no need to go off the road to pick them up, and there are even a couple items near obstacles that you should just ignore. (Note that item pickups are valuable in the Mexico 250, though, since you only get one chance to upgrade your truck.)
If your character is saying "¡Arriba!" a lot, you may be going over hills too fast. Sometimes speeding over hills is warranted, especially when you're rushing to catch up with Ivan, but often you'll want to take it easy over hills because you'll take fall damage and you can't see where the obstacles are. Speeding over hills is also needlessly risky in the first three races of the Ensenada 500 and Baja 1000, since there is no need to keep up with Ivan until stage 4.
Use of nitros
Use nitros:
- To get back up to speed after slowing down too much.
- To quickly get back on the road if you're way off on the side.
- To catch up to or pull ahead of Ivan.
Don't use nitros:
- During the first three stages of the Ensenada 500 or Baja 1000. The speed boost is not necessary. It's OK to use one now and then when your speed gets drastically reduced if you're impatient, but there's no tactical advantage in doing so.
- To stay ahead of Ivan (unless you're on the final stage). There's no need to since you will never get far ahead of Ivan, so trying to hang on to the lead will just make your car hard to control.
- While turning. It's best to fire your nitros on straightaways so you won't have to turn and potentially lose control of your truck.
- Going over hills, unless you're desperate to catch up to Ivan (such as in the second stage of the Mexico 250).
- If you're off the road and almost back on it. The nitro will be much more effective on the road. (This advice is most relevant for the Mexico 250, where nitros are in short supply.)
The AI
It is not particularly useful to stay ahead of Ivan. The game's "rubber band AI" means he will never be far behind no matter how fast you go, and he will ram your truck repeatedly, making it hard to control. Thus you should usually aim to stay right behind Ivan (though it's OK to pass him now and then if he's in the way) and only try to pull ahead of him towards the end of the final leg.
The rubber band AI also means the AI will slow down if you get too far behind. For instance, Ivan's total time at the end of stage 2 might be 5:30 if you finish both races quickly, but it might be 7:50 if you took a very long time to finish stage 1. However, it does not seem possible to exploit this to make the race easier to win.
The races
Ensenada 500
This is certainly the easiest race to win in Expert mode, which is why it's listed first. Stage 4, which is the final stage, is so easy you can play it with your eyes closed: just buy as many nitros as you can and fire away. There are so few obstacles on stage 4 that you cannot break down unless you are doing it on purpose. The main thing, then, is not to break down on the first three stages. There is no need to catch up to Ivan until the final stage, at which point you will very likely do so no matter how far behind you may think you are. Thus, go as slow as you need if your damage is high. You might even want to let all the cars pass you and drop into last place (13th) so they can't knock you into things. If you do, you'll have to work a bit harder on stage 4 than usual, but catching up is still not difficult.
If you often find yourself breaking down in stage 2 or 3, you can try not upgrading your engine or even selling it all off until the start of stage 4 (where of course you'll want to max it out), since your lower speed will make it much easier to avoid obstacles. If you do sell it all off, you're virtually guaranteed to make it to stage 4.
Baja 1000
This race is comparable in difficulty to the Mexico 250, but much longer, and possibly harder. We'll list it second since the strategy for the first four stages is exactly the same as for the Ensenada 500.
Again, you will catch up with and probably overtake Ivan in stage 4. It's the next half of the game where things get serious. After stage 4, do not let Ivan out of your sight. You will never get an easy opportunity to catch up with him again. If all you care about is coming in first place, you have to take risks. Don't drive carelessly, but keep plunging ahead even if your damage is a little high, since playing conservatively is a recipe for coming in second. Also remember not to spend too long in the shop between stages. Don't waste time buying nitros if you already have 40 or more, for instance.
Buy lights in stage 6 or 7, whichever you prefer. The difference in visibility is noticeable in stage 6, but not important until stage 7.
Obstacles are dangerous throughout most of the stages, but be especially careful on courses 7 and 8. A couple of hits might be all it takes to seriously jeopardize your chances of making it to the finish line.
Obstacles get downright murderous starting in stage 7 or so. Be extra careful not to hit them.
Stage 8 is tough. It will take everything you've got to keep up with (let alone stay ahead of) Ivan and not break down. Good luck!
Mexico 250
On Expert mode, no matter how good you are, you won't even see Ivan until stage 2 because he starts with such a massive lead. Still, do your best so that you won't start out too far behind him in stage 2.
Item pickups are much more important on the first stage than usual, since you'll definitely want maxed out engine, shocks, and tires and a decent number of nitros. Money pickups are useless on the second level since you can't spend the money, so ignore them, but nitros might still be handy if you're running low. In both stages, be judicious in your use of nitros: don't fire them if you're likely to just slam into a buggy, truck, tree, etc. and slow back down, but do use them whenever you're going too slow if you have room.
Be warned that, even though this race is only two stages long, beating Ivan is extremely hard on Expert mode. It's a very common outcome to never even see Ivan, let alone pass him. Still, it is possible. Good luck!
Time limit
If the clock ever reaches 7:39, you will be disqualified. The exact time limit may or may not depend on which leg you're on, but in any case, this will never happen unless you do it on purpose.