Acknowledge that higher skill levels will benefit you the most. If you fish, farm, or gather your own stuff, you will make more profit rather than buying the resources.
Study the Grand Exchange prices. Know what is in demand, when there are losses on an item, and what you can gain on an item.
Buy what you can do or wanting to focus on. Don't go off and buy a huge amount of something and can't finish it. You also don't want to buy a huge amount of something and find out that prices changed unexpectedly. Doing this will make you lose money instead of gaining it.
Use cooking gauntlets as much as possible. You will need to finish the "Family Crest" quest, but it's worth doing so. You'll not only be able to lessen the amount of fish you burn (if you don't have the correct cooking level to stop burning them), but you will never burn rocktail, shark, and cavefish after a cooking level of 94.