Don't Give Up
Even if you don't get the first grant you apply for, that doesn't mean your idea isn't great or worthy- just that it may need refinement or or your application may need help. Take it as a chance to refine your idea or work on your application and apply to another opportunity! On the same token, don't be afraid to scrap an idea and go back to the drawing board.
Meet Follow-up Requirements
Many grants require some sort of follow up. Some require you to submit data or a final report. Others want copies of documents created or thank you notes. Be sure you know what the follow-up requirements are and meet them. Failure to do so will often make you ineligible for future grants.
Stay True to Your Grant
No matter what curveballs come your way, be sure to stay true to the original purpose of your grant. Some adjustments might be necessary, and that's usually OK. But, buy the equipment you said you were going to buy, and do your best to use it in the way planned. Failure to do so can also result in not receiving funding from that foundation or grant source in the future.
Best wishes in your search for grants!