Much has been written on the subject of making gold and of course whole blogs have been dedicated to the subject.
Once upon a time the Goblin and Wowenomics were two of my favorite gold-themed sites. The former has generally stopped writing about making gold and the latter has unofficially closed it's doors (last post Nov 09). Wowenomics has a lot of really good links still to point towards a large number of money-making sites.
Currently I visit Marcko's site quite frequently to see what fresh gold-making ideas are out there. Contrary to what you might expect, however, I actively discourage taking the advice of MMO-Champion or Wow.com on these topics, as they're so widely read that their advice tends to move markets to the point of unviability fairly quickly.
At any rate, if you are wanting to get into the Rockerfeller category, go to those other sites and do your own extensive research: my topic today is about making - for starters - enough to cover all the expenses of raiding semi to completely hard core.
1) Profession Daily/CD. Most professions have a daily or a transmute/cooldown. Min-maxing DPS DKs are fairly likely to be jewelcrafters/blacksmiths, and the jewelcrafting daily is a very quick and easy source of 120-180g in 15 minutes or less. Cooldowns and transmutes tend to be worth a little less - about 30-50g, but still worth doing.
2) Daily quests. There are several daily quest hubs that allow one to quickly make money. Hodir and the Argent Tournament are perhaps the best known. I've read and seen vids on several sites timing the runs and gold made, and it varies from 200-300g per 40-60 min. That's not terrible, and certainly would provide enough gold to maintain the mundane raid expenses, but I don't prefer the grind that it leads to after days or weeks, and ultimately it's not as efficient as other methods.
3) Daily heroic. As I mentioned last time, the daily heroic is an easy source of cash. You need it while building up your emblem gear. Once you have all the emblem gear you could possibly want (as I do at this point), you can start selling primordial saronite for a pretty penny. Depending on your luck with pugs or guild groups, listing yourself in LFG and running the daily can take from 20-40 minutes, and nets about 250-400g. I specifically keep a tank spec/gear or now so that I can have instant LFG queues.
4) Professions. Your mileage may vary, depending on your server and profession. In a recent post at Just my Two Copper, some have suggested you can make your full 1k on JC alone. I generally list 15 (weekdays) to 25 (weekends) cut gems on the AH daily. My average profit margin is about 30g, for about 450-600g a day. This sounds good, and it is - it only takes me about 20-30 minutes daily to check the AH several times, buy uncut gems, mail them to my DK, cut them, mail them back, and list. I only list one epic gem of any given cut at a time, put a second gem up if I'm undercut three times, and list for 24 hours, letting unsold auctions expire. I list at consistent prices; never more than 40g above the 'normal' AH price even when the market is empty, and if prices drop below 20g profit, I won't make or list any gems of that cut until the margin returns. I list nightmare tears occasionally, or make epic rings/armor to sell my Dragon's eyes, but I don't sell them all. I list a few meta cuts a day. All that takes me about the same time as it took to write this paragraph. Every once in a while if the uncut market is too high, I'll either wait it out, or turn in badges or honor to get a few uncut gems to replenish my supply. This market requires a certain amount of operating capital - about 4-5k gold just to keep a decent stock of cut gems, but it pays well.
I know my market; if you get to know your 'main' profession well, there is profit there. I don't bother with blacksmithing, but I know folks out there who make hundreds a day easily in this market. Inscriptions is famous for the rail barons of moneymaking. It's cheap to make and list scripts, so the savvy investor follows the market, knows where the profit margins are, and lists dozens a day, not worrying much about items that don't sell. In any case, the picture here is that it is relatively easy to use your profession along with various dailys (quests/dungeons/etc) to make it to 1000 gold per day.
5) Still need more help? Auction House brokering is a way to make HUUUUGE money; I only dabble in this area, but the gold-cappers are masters of this technique. Again, there are whole blogs devoted to this topic. If you need help, read them. Get Auctioneer, or AuctionLite, or just wait for the next patch which will make the basics much easier. I have about 15 items that I know the market fairly well; their ups and downs, and their cycles. I'll just list one for an example. Damaged Necklace is an item that consistently sells for 320-350g, however, it fluctuates a lot. I usually buy it whenever I see it up for under 280g. Recently 20 or so were up for 250g or less. I bought 10. Concerned about the risk of the bottom dropping from the market, I sat on those 10. Prices stayed low for a while, but they eventually returned, and I've sold them all for 325-350 again. Abyss crystals often appear on weekends for ~24g, and I always sell them for 33g or more during the week. I also buy low sell high on other enchanting mats, cloth in bulk, eternals, and a few other items.
The above only scratches the surface of gold-making in wow. There's hundreds of additional little tricks and strategies to make nearly unlimited amounts of gold. In any case, I hope you can see that making 1k gold a day is very possible, without grinding or farming for hours a day. If you can hit that 1k benchmark, you'll be set not only with repairs, consumeables, and item enhancements, you'll be able to afford the crazy-expensive crafted items that will maximize your performance in each new tier of content.