If you're new to writing arts grants, I absolutely recommend thoroughly reading through Kate Larsen's resource:
Tips for writing great arts grant applications
My top three things when I write an application are:
Clarity
Relevance
Viability
Kate goes into all of these and more, summing it up perfectly.
Please, if you're new to grants or you're struggling to get successful applications, read over Kate's points carefully - highlighter and pen in hand!