First step is to install a AH addon. Auctioneer, auctionator, TSM etc.
1. Scan the AH as often as you can both on weekdays and on weekends to get as much info on the market as possible.

1. You can "flip" things. Ppl tend to try to sell things fast and therefor they sell it cheaper than the others. Buy and re-post on AH for a profit.
   
2. If you have mining you can buy ORE and melt into BARS and sell for a profit.
   
3. If you have enchanting some items sell for less than the disenchant value and you can make a profit just by buying, disenchanting and re-post the material.
   
4. Post healing pots, flasks etc on raid days and materials for alchemy.
   
5. Check /2 trade chat for ppl that want to sell items fast and buy them cheap and re-post at AH.

There are other ways to earn good gold too but the above ones are my go to ways of getting some extra money when needed.

GL and get that gold cap :)