Get the bid/contract in writing (email).
Shop around.
Make sure you have 4WD (small one is fine).
GPS can be very helpful, but the ones that come with rentals add a lot per day. Might be better to have your own.
When you get quotes, make sure you waive collision; your credit card will probably cover it, but check with them. You will have to pay the liability. Make sure you have a letter from the credit card company stating your CLD coverage when you show up for your car.
Print out everything, and make sure that you verify your contract price before you get the e-mail and print it.
Do a COMPLETE inspection of the car, and take photographs of any damage, however minor.
Be sure you have a spare and a jack.
We've had good luck with Solid and Europcar.
Other tips:
don't add apostrophes to plural words
it's not "me and my friend we wants" it's "My friend and I want"
Have fun.

Hi, thank's for the advice. next question :
There are so many rental car companies around, which one I should use?
Cheer's
Kevin

In addition to sylvia3 tips. in case of waiving the CDW, a hefty deposit will be put on your C/C; in case of damage you pay first and then ask compensation later.
Majority of rental car companies have their fair share of issues: pushing the insurance, charging for minor or pre-existing damages, not having the car that was originally booked, etc. Big boys are franchises only so not much more to expect from them versus the local companies. A small one can be as good or better then a bigger one since their main advertising is mouth to mouth type (on forums). You may want to check the Tripadvisor forum as there are more posts related to your question. saying all this I have rented twice from Wild Rider www.wild-rider.com, a small company specialized in 4x4 (and bikes) and have both time excellent experience. I've paid exactly as it is advertised on their web site, cars were older but maintained well, customer service first rate.

The deposit is not charged by everyone, but be ready for it...shop around, due diligence.
We've liked Solid (out of LIR) and Europcar (SJO). Mapache also has fans.
But as xelas says, don't think that "Avis" or a big boy name will be any better than anyone else.
You should be checking websites now (get preliminary ideas from hotwire, etc).
(Do not think to arrive in the p.m. and start driving somewhere; that would be a huge mistake.
Most places will deliver your car to your hotel early in the a.m., if you are close to the airport.)

Has anyone used POAS. Someone recommended them tous.

I havent used POAS yet, I had very bad experiences with Mapache, Europcar and Thrifty.
About small companies, it is difficult to say, it is not the same to rent with a large multinational company than to rent with a small local company.
I always call Dollar, there is a guy Diego, always good prices with no hidden fees.