The purpose of carry on luggage is to give you access to the things that you may need in flight, not to save yourself 15 minutes in collecting your checked luggage when you land.
Pack any medicine you may need (minimum of a 5 day supply), a book, and maybe a change of clothes in case you end up in Sydney and your luggage decides to go to London. You'll at least have a change of clothes until the airline finds your luggage. Also, keep any valuables you may have in there instead of your checked luggage.
The airlines are becoming very strict on this matter: Your carry-on has to fit in the over head compartment or you get to check it. They are getting tired of late take-offs while some jerk tries to cram a refrigerator-sized bag in the overhead, only to discover that he has to check it anyway. (See baggage limits below.)
Most airlines now have a template on the X-Ray machine that your bag has to fit through. If it doesn't fit, sometimes you can check it there, and sometimes you have to go back to the ticket counter and wait in line all over again to check it.
Always take a larger suitcase than you think you'll need. By the time you get all your souvenirs in there, you'll thank yourself. That means you won't have to buy a second bag for the new stuff. (See baggage limits below!)
I've fallen in love with the bags with the wheels on one end and a collapsible handle on the other. Just make sure you get a good quality one with metal wheel hardware and not plastic. Plastic won't stand up to airline baggage handlers. You'll pay more for it, but you won't have to replace it after the first trip... or before you start home.
Count on every bag on the carousel looking just like yours. Make yours easily distinguishable with a large gaudy luggage tag or some colored cloth tape. I saw a guy in JFK who had put his initials on the side of his bag with silver duct tape. Ugly, but effective. There wasn't any doubt whose bags they were. Great idea!
Lock your bags. The lock won't stop thieves. They'll just take the whole bag, but you may not have to sit on the terminal floor and repack after the zipper works open in flight. I've seen it happen. (And, trust me on this one, the floor in Toronto was COLD!!!)
This was copied directly out of a reservation I made on Continental Airlines:
BAG LIMITS STRICTLY ENFORCED...
2 CKD BAGS... (checked bags)
1ST MAX SIZE 62 LINEAR INCHES /70LBS...
2ND MAX 55 LINEAR INCHES/70LBS...
TOTAL CARRY-ON MAX 45 LINEAR INCHES/40LBS.
A Linear Inch means that you wrap that measuring tape all the way around the bag. Take a saw to anything over 62"!! Or get a smaller bag. They are serious about the weight, too. Dust off those bathroom scales!
Yes, they can enforce it!! It's THEIR airplane!! They don't always charge extra if you don't meet the requirements... sometimes you just can't take the bag... it's that simple.
If you have something large (or heavy) to take someplace, consider asking the airline ahead of time about air freight.