Low level mischief, whistles and hatchlings

Here's a tip that is pretty well known, but which effectiveness would vary from server to server.
It has to do with listing vendor sold pets on the AH for profit.

The first is the prairie dog whistle, which you can obtain from thunderbluff. If you park your bank in thunderbluff like I do (since it is much less laggy than crowded orgrimmar) its a vendor pet that is extremely easy to obtain, costs 50 silver, and would sell for 5 - 10 gold on the auction house. It sells because the extra few gold premium that buyers pay for it is equivalent to the opportunity cost of them making their way to thunderbluff all the way from undercity or orgrimmar.

Another good place is Eversong Woods, where you can buy the different dragonhawk hatchlings. This particular trick will take a bit more work, since you must run quite a bit from silvermoon city (another good place to park an alt) to fairbreeze village, and then back again to relist in on the AH. Alternatively, you could simply roll a new blood elf and run it to the village, and simply mail it to your bank alt. The dragonhawks costs the same, but can sell for slightly more, sometimes even for 15 gold.

What you can also do in addition to these, is to bring those two pets to the neutral auction house in booty bay. Since these are horde specific, alliance players will also pay extra for access to these pets. Combined together, the effective marketing and selling of the pets is a good way to make gold at both low and high levels.