Do your homework. This includes consumer magazines and price guides, and checking out the Internet. This will help you decide what features you want, what specific cars fit your needs and what price range to expect. Decide ahead of time what features you want in your new car. This helps avoid being talked into options you may not have budgeted for.
This is especially important if you are buying a used car, which typically have no warranty, but should not be ignored with new vehicles, either. Learn everything you possibly can about the vehicle you are buying - including reliability, repair history, maintenance issues, accident records, and insurance costs (whether due to accidents or theft). You should do an internet search for the car name plus "sucks" which is how many consumers vent their beefs for shoddy vehicles.
You should also research the dealerships from which you are considering the vehicle purchase.
Contact your local Better Business Bureau (BBB) for some insight with a reliability report on the dealership. Inquire with provincial Consumer Protection departments to see if a dealership has had any complaints filed against it.
Use an evaluation sheet to keep track of the cars you check out. Jot down what your inspection of each auto reveals. Watch for anything that is different from the research you have done. It will come in handy when comparing the pros and cons.
Don't "fall in love" with one car, to the exclusion of other brands and models. Evaluate several cars in which you might be interested. You lose a lot of negotiating power once you're set on buying one particular type of car. This way, if one car isn't in your price range, you can focus buying effort and negotiations on another car. Lastly, never buy a used car (or a new one either) on impulse. There are many places to shop for a used car, don't just stop at the first one.
New doesn't mean it's better. Avoid buying new models in the first year. Almost every new model has some unanticipated design defects (just like computer software… wait for version 2.0).