Ask for what you need
If you don't need it, don't ask for it. The magical power of a spell is driven by a true need; if you're asking for a second yacht, you probably won't get any results that you would like.
Don't fence yourself in
The most likely reason for a spell to fail is if you're asking for something too specific - you want to give yourself and the universe as much room to maneuver as you can. Don't focus on details, focus on the feeling you want to have when things work out right. Close your eyes and try to find that feeling "out there;" if you find it, either move toward it or reach out and bring it toward you.
Pay attention to the tides
The second most likely reason for a spell to fail is if the time is just wrong. Remember that spells work within the bounds of nature, and sometimes the wind and the tides are working against you. When that happens, the best thing you can do is to put a finger to the wind and figure out where you can go, working with the natural flow of things; or just wait it out until the tides shift in your direction.
Give something back
If you're asking for something, it's a good idea to offer something in return, at least as a token gesture - a flower, or a bit of food, or a coin, for instance (no live animals, please).
Remember other people are using magic too, whether they know it or not:
Most people have some natural magical ability, whether they know it or not, and an instinct for taking care of themselves. If you try to cast a spell that is inherently detrimental to another person, you will meet resistance from that person's magical energy. The magical universe is a crowded place, and things will work out best for you if you manage to navigate a path that avoids direct conflict with your fellow beings.
Follow up with mundane action
Another important point about spells is that it's very important to follow up with mundane action. If you cast a spell for money, don't expect a check to just show up in your mailbox the next day; get out and look for a job, or at least buy a lottery ticket (magic doesn't work terribly well with those, by the way, because everybody who buys a lottery ticket throws a bit of magic into it, whether they know what they're doing or not). The idea is to make a point of putting yourself into situations where the magic has a chance of working.