Staying on budget when decorating requires discipline. About.com's tips will help you make a budget and stick to it when it comes to home decor purchases.
Accordingly, your first step is deciding on a budget. Your budget is how much you want to spend, not how much you decide you want to spend after you get to the store and see those adorable hand-embroidered throw pillows. Along with your first step of planning, your budget, you need to come up with a decorating plan. Before you go out, have a good idea, and even a physical plan, of what you want your room to look like. Ask stores for samples so you can test them in your space and make sure the store has a good return policy if you wind up not liking your purchase in your space.
Now that you've got your Plan A drafted, go ahead and put together your Plan B. When getting ready for your project, always assume that everything will take longer to complete and cost more than you expect. That way you can be prepared for any cost or time overruns that come your way. You'll also be ready if once you get into your project, you realize you don't have the funds left to buy the curtains you'd wanted to match your new couch and may have to live with the old ones for a while. And since you already have the discipline not to go over budget, that won't be a temptation, right?
Finally, know when it's time to call in a professional. Do it before you've made the mistake that will cost more to fix than to just get it done right the first time. Above all, never give up on finding what you want for the price you want it. Keep looking and eventually the perfect piece to complete your apartment will come your way!