I can't help much because I just visit 1 or maybe 2 countries per trip in the 4-6 weeks I have each summer. To start with, you can use US currency in Belize and Panama so that makes 5 countries to deal with. You might want to pick up Mexican pesos at the airport in the US to get you started and use ATM machines for the others as you head south. It will be a bit of a challenge to exchange the Guat, Hond, Nica, and CR currency when you get home so don't leave with a stack unless you want it for a future visit.

Be sure to contact your debit and credit card companies to leave your dates and places of travel so their fraud alert departments won't freeze your accounts while you're on the road. I also pack along some travelers checks for backup. Good luck!

Every time you reach a country use a local ATM at the airport (except Panama and Belize unless you are out of US cash) to get local currency and (except for the first country) change the previous county's money into the new country's at a money change desk. Don't return home with any appreciable amount of foreign tender as it's almost impossible to change it back to US once in the US. Don't bother with travelers' checks as it's getting harder and harder to find anyone outside a bank to cash them.

If you change money at a money exchange in an airport, keep it to minimum -- they're well known for having the worst exchange rates.

Use ATMs, but be careful with them -- I know a couple of people who had their cards "skimmed" in Mexico & Guatemala. ATMs inside banks are probably the safest.

In Mexico, money changers are common -- you'll see them on main streets.

In Belize, use can use US$, but you'll usually get BZ$ as change. US$ also used in Panama.

When you leave a country, get rid of the currency of that country. If crossing at a ground border, there are almost always money changers hanging around the immigration buildings.

Here's my standard reply regarding Belize...

US currency is accepted in Belize. Bring lots of small notes ($1, $5 & $10) because sometimes the larger denominations might not be easy to change. When quoted a price, make sure you also know the currency.

When you spend US currency, you’ll probably get BZ currency in change. Make sure you use all your BZ$ before leaving Belize because it can be difficult to exchange outside Belize.

ATMs are in every city and main town. They dispense BZ currency.

I use a plastic card to get money from an ATM, but still carry a few traveller's cheques when ATMs are out of order or out of money (yes, it happens), or any other problems with the card (saved me on a few occasions). Never had a problem cashing them and never had to wait long at a bank. Often the queue of people waiting outside in the hot sun for the ATM is longer than the queue inside the cool air-conditioned bank. Whatever people say about traveller's cheques, it's still a good way to carry money.

Don’t forget to notify your bank that the card will be used in another country.

Agree with misterb and nofly and would add that if you are crossing borders by bus you will find guys changing money at the borders. Exchange small amounts to get you through. Use small denomination bills, no one will have change for large bills. ATMs are available everywhere but use bank ATMs within the bank for security.
I carry two ATM cards from two different banks (Not Debit cards) in case of trouble with one. I like to have some USD in twenty and tens, rather than travelers checks, seems USD still has clout.
Contact your bank and cc company a good tip from hopeful

As per Belize,

Everywhere in Belize accepts US currency. The exchange rate is easy: BZE$2=US$1. You would more than likely get Belizean currency as change, but don't worry if you have a lot of Belize currency cos if you are heading out to Guatemala after Belize (more than likely), you can always change the Belizean currency you have left at the Guatemalan border. A lot of persons do money exchange at the border. You would get approximately 3.5 quetzales (guatemalan currency) for one Belize dollar. There, they can also change your US currency for quetzales, which should be the double what you get for a Belize dollar. Hope this is helpful

For the non-dollar countries (Panama in this case), it's almost impossible to exchange their currencies outside their borders, even in a next-door country. As mentioned above, there may be a few guys hanging around at the border itself who will do it for you, but no bank, say, in Nicaragua, is going to touch your Honduran or Costa Rican currency. For each country, try to spend down your local currency as much as you can before you leave, or change it back into U.S. dollars.

That should ge:

For the non-dollar countries (all except Panama in this case) ...

In Mexico you will want to have peso.

Especially since you'll be there several weeks it is worth it to change your own money (at a "casa de cambio") or better yet use your DEBIT (not charge card) card to withdraw peso from a bank ATM machine.

Even if merchants accept USD (which they will especially in tourist areas) likely they'll give 10/1, when today's bank rate is more like 13/1.

Fantastic! Thankyou all so much for the advice - it really is appreciated.

Our government (Australia) just released a new security warning (includes withdrawing money from the airport (MEX). While I know, they can be over the top I don't really like being too complacent about my safety.

"There have been a number of incidents involving travellers changing money at Mexico City airport. Gangs are known to have arranged attacks on travellers at the airport after they have changed money. You should avoid changing money at the airport if possible or change only small amounts to avoid attracting attention."

So thanks for all your help - advice well taken on board!

Don't worry too much about the security warnings. Yes, things can happen, but most people who visit Latin America have no trouble at all.