Long time lurking, first time posting so sorry if I babble incoherently. I have been playing WoW for about 8 months now and I have levelled crafting on all of my toons. This game is great for crafting if that is something you enjoy doing in your mmorpgs, nothing like the crafting system in FFXI. I recently dropped Skinning in favor of Leatherworking on my rogue and I have levelled it from 1-340 with about 100G spent. The best thing to do with any profession in this game is watch the server economy. Craft things that sell well or that cost little in terms of materials. For engineering I made all the blasting powder I could and gathered the mithril and thorium I needed with one of my alts. For leatherworking I had skinning and gathered all my materials that way. Crafting doesn't have to be expensive or time consuming, you just have to be smart when you are deciding what to make and when you are going to make it.
If you look at a guide and it says to make one thing make sure thats the best thing you can make for the level, allakhazam, thottbot and wowhead have lists of the crafting recipes by level and they show the maximum level you can get to before something turns grey. Remember that just because something is orange doesn't mean it is the most cost effective thing to make. If you look on the list of recipes that are available and something that is incredibly cheap will get you just as high as something that costs a lot of gold don't waste your money.
If you have the time, make a gathering alt. I have a hunter that does all my gathering for me (skinning/mining) as its a lot cheaper than buying the materials all the time; though I do often buy the materials because I have the gold and get lazy sometimes. Another thing I usually do is mail excess materials to an alt if I am not going to be using them. They stay in the mailboxes for 30 days before being sent back so thats 60 days you could have something floating in limbo and not taking up space in your inventory or bank.
Lastly for now, when you are picking your professions make sure they are something that suit you. If you really want a good piece of boe gear then go for that profession. If you think theres a really cool item that you can make with a certain profession (i.e. world enlarger) and you really want it, take that profession. Its your game, play it how you want and don't let other people tell you what professions you need. I really wanted the cool looking goggles with engineering and the gyrocopter so I selected that profession. Leatherworking had some great armor for me at lower levels and was a decent source of income (at times) at higher levels. Also if anyone wants me to post my personal guide for how to get from level 1-375 with leatherworking, engineering, tailoring or enchanting let me know... I have spent time with all of them on the few characters I have.