Liz, I have found most recently that a real good tip is to make absolutely sure whne you're doing perspective and aligning up things like reflections that those things are PERFECT in alignment. I trace right thru my photos on a window. DOn't loose those pencil marks and think, oh, it's off just a litte, but it's ok.
The rest will take care of itself as far as color/value. The other thing I've learned the hard way is that COVER UP THE CANVAS. Don't put anything in until you've covered everything up. But work around the things like windows, etc..(i paint in the outlines of things like this with paint first then dry). Doesn't matter if the outlines are inaccurate..it's just so you won't loose site of them. Windows? FILL it in. Then, go back and do the panes, the cross frame etc. Even now I made that mistake at first, but realized that everything even them...fill in first. Don't ever work around things like making the cross frame of the window first...detail is always last. This whole concept works, and it helps put your mind at ease too because you see progress being made fast.