There are a lot of skills and spells you can train as you progress, each of which costs money. When you can afford to, you should train all the abilities that your class trainer offers. If you're completely broke, it's fine to put off upgrading abilities you rarely use for a level or two so that you can upgrade your most-used abilities. If this happens, you should ask yourself whether you are spending too much money on buying unnecessary equipment upgrades or leveling production trade skills. Be sure to save enough for class abilities and riding training before spending money on other things. This assumes you're earning money at a rate where buying skills makes a difference at all. If you have a hundred gold from two gathering professions by level 20, those skill ranks costing a fraction of a gold won't make any difference to your purse.
These are Warlock- and Mage-only skills which can net you some money. It's not much, but a few gold is worth it. Note that Mages have an easier time with this than Warlocks as Warlocks need to be at the location for the summoning and need an additional person (a 3rd) for the group to help out with the summons. Mages only need the reagent to open the portal. While the Warlock doesn't incur a cost (except the loss of a soul shard), the Mage will typically be more successful at finding employment using this method.
Hey Bunnycuddle, big fan of your approach to gold making. Due to recent drop in obliterum market prices i have started exploring the best flipping possibilities out there. I found one very interesting phenomena that proved to be very profitable on my server. Basically due to it being least desirable gem Versatile maelstrom sapphire tends to drop in price on AH well under 160g per piece, and while oblierated it gives 16 ash in return. I think this is something you could possibly research in your future post and give a little bit more in depth analysis.
If i start on a new realms, I am farming a lot of transmog from classic and Burning Crusade Dungeons and Raids. In the beginning, i start to post them if they have at least a DBRegionMarketAvg (Average value of an item on all EU realms of the last 14 days) of 500 Gold. All the other items get vendored. The time you hit a stock of 1000 Items, my value treshold raises to around 2.500 Gold. All the items below that value get vendored aswell. Newly farmed items that are above these treshold will be listed. Why? If I start over somewhere, I want to get a certain amount of gold quickly. Cheap items are great for that. Later on, to save time, Quality is better than quantity. As mentioned above, re-posting the items on the auction house takes quite a bit of time. At this point, I can recommend you the TradeSkillMaster AddOn, it makes your auction house work really a hell of a lot easier.