After being in this business for 8 years (I'm the Airport Operations Manager for National and Alamo in Halifax, Nova Scotia) I've picked up a few things. Some points to consider:
~ Rates are tiered, from the bottom up, in the order: rental companies own websites (alamo.com), travel websites (expedia.ca), 1-800 reservation center lines and then local numbers (right to the local offices). So hertz.com or nationalcar.ca will usually be the cheapest place to start.
~ If you're a member of ANYTHING, mention it when making a reservation or inquiring about rates. Aeroplan and other frequent flyer discounts can take 10 or 15% off local rates, a Costco membership with Alamo can waive an additional driver fee, and you'd be surprised who has a corporate account signed up (I only found out a few weeks ago that The Playboy Corp. has an account with National! If only they'd sign up Chippendales.....)
~ Most car rental companies don't have a lot of economy cars in their fleet - so if you do book one, you most likely will get bumped into a LEAST a compact.
~ Lots of people book the smallest, cheapest thing available - but take a look at what you really NEED. If there are four people travelling and luggage, you'd better at least be looking at an intermediate or fullsize. No matter how much you think you saved, being cramped and uncomfortable isn't worth it.
~ Most rates differ between regular car classes by only .50 cents/day to $2/day. Over a week jumping two classes from a compact car to a fullsize, that's only a difference of $24 for the week. I'd rather be comfortable in an Impala or Five Hundred rather than a Cobalt or Focus. Factor in how far you'll be driving, how many people will be with you etc.
~ UPGRADING at the time of rental. Don't expect a good cheap deal on a convertible in July or a 4x4 in February. But at times of excess capacity, you may get that premium car for only $9/day extra or a minivan for $12/day extra.
~ As said before, prepaying with a tour company is great, especially coming from Europe. Alamo and National do a HUGE amount of tour business and most customer have told me that prepaying did save them LOTS of cash. Plus when you arrive you're not having to worry about paying a large bill when you return as it's been paid before you leave. And go for the best package; with Alamo it's usually called Alamo Gold and National's are usually Fully Inclusive, Enhanced Fully Inclusive or Super Inclusive.
~ Remember, you can only reserve a car class, not a specific make and model. My reservation centers are famous for informing customers that they are guaranteed a red Sebring with aluminum wheels and a black cloth interior. When they arrive and I have a green G6 with a beige interior that FREAK OUT. So if you book a full size, there are 6 or 7 models that you MAY get. Best thing to do is after you make your reservation, ask for the local number of the office where you'll be picking the car up, call them and inquire about the vehicles in their fleet.
~ Asking for an upgrade at a car rental agencies is the same as asking for one on your flight. Is usually doesn't work. We've heard all the excuses before. If you and your family of four have arrived in February with ski gear and you're booked for a full size and you whine for a sport utility - you'll get one - but only after paying a HUGE upgrade rate.
~ just walking up to the counter and asking for a rate sometimes works - I set my walkup rates each day depending on the number of cars on ground, utilization, daily transactions etc. So I've actually set my premium cars lower than my economy cars just to get them on the road. Just don't arrive without ANY reservation at all. Inquire at the desk and if you have a reservation with them, inform them that you do have a reservation but their walkup rate is better and could you please process them as a walkup. 99.9% of agents will.
~ The WORST things you can EVER say to a car rental agent:
"You're the only one who has cars left". Yes, and now you're going to pay dearly.
"Do you have any (insert specific make and model here"). If we do, you're also going to pay dearly.
"I think I'm going to need a bigger car". Saying this means you'll pay for more. Wait until the agent asks you and then you can sometimes negotiate on price for an upgrade.
It's not rocket science to anyone but car rental agents get paid commission on any optional items sold at the time of rental. Just make sure you have all your bases covered and you KNOW your stuff.
Hope I've given you some good tips. If you have any other questions IM