Try not to have your main character have two of anything to keep track of. My PI drives two cars--one, nondescript, for business, one for pleasure. But it got mixed up in there when she loaned her business car to her aunt. I didn't keep good track of what she was driving, and now I will have to search the whole document and be sure it's the right car!
Thank goodness for the search feature in word processors.
Pick your character's names with care and try not to change them. Yes, the search feature will work here, but if you use certain names, the universal search and replace will put them inside words or at the beginning or ending, making for strange new combinations. Easier not to have to correct that.
Read the ending--last quarter or third-- several times. You've probably read the beginning over and over again, but as you get further in, you will probably read the later stuff less. Make sure everything makes sense at the end.
Print it out. I bought a ream of cheap paper and printed the whole manuscript, then used my red pen. I corrected everything from that first go-through on the computer, then printed it on the back of the first printing, after making a red slash through the other pages so if I dropped them in a heap, I'd know which pages were the newest ones.
Use the page break feature in your word processor at the end of each chapter. Then each chapter will start on a new page--nice.
Have lots of room for three piles of paper and for a notepad. You have what you're reading in your first pile, what needs fixing in another pile, and what's okay in a third pile. Make notes as you go through--during the first run-through you will probably find several things that need fixing on pages you've already edited, so you will have to go back and find the spot(s) where you need to make adjustments. But don't try to do it during the run-through itself.
Plan for large blocks of time with few or no distractions.
Take your time. After the first fix, let it sit awhile and mull things over. You may need to do this more than twice--I
ended up doing three print-outs, read-throughs and edits. Each edit took me part of a week, but I didn't begin on the next one until the following week. I think this time is needed, especially if you don't outline, to be sure all the pieces are in place, all questions answered, all details correct.