As weird as it sounds, I treat Magic as a bill each month. I have so much budgeted for it. A few things I did (and still do for the most part) before I was financially better however....
1- Despite the thrill of opening packs, it is usual cheaper to just buy the cards you specifically want than it is to buy packs hoping to get the card you want.
2- Trading, most likely there's going to be people that want the cards you have extra of that have cards you want...unless you're all building the exact same deck, which would be a little awkward. The bulk of your deck should be commons & uncommons. Those rares you do have from packs you opened that aren't in your plans for building decks with are your key towards trading for the stuff you're looking for.
3- NEVER preorder individual cards, mainly meaning rares and mythic rares. Some may argue with this but they can't argue that the majority of the time the price of individual cards drop once the set is released. To give you an example, the planeswalkers from SOI. Before the
release Jace was preordering for around 35-40, Nahiri was around 30, Sorin was around 35, and Arlinn I saw as high as 45. Now, none are more expensive than 30.
4- If you are going to purchase cards online, be mindful of where you shop. Some sites charge more than others just like with anything else you buy.
That's all I can think of off the top of my head.