Also if you are getting streaks and patches and it is something like a flat panel you are painting then use a bigger brush.
I actually had a conversation in a GW store the other day with a member of staff when I was buying brushes. I bought a Standard and a Detail brush and the staff member just in passing said something along the lines of "I only ever use a fine detail brush personally..."
Probably after ten minutes of painting his FD brush looks more like a drybrush than anything else, still being dedicated to the hobby doesn't mean that he had to be any good at painting did it?
Use the right tool for the right job, if it is a large area use a large brush and the paint will cover more evenly without little tickle marks and without you having to damage your brush by overloading it. Remember there are very very few reasons to use a FD brush for anything slightly more reasons for using a detail brush and for average everyday painting, shading and highlighting a Standard brush is, well standard....