Most important (other than specifying veteran's preference, which you should definitely do) is to be sure to tailor your resume for each position. State your previous experience in terms of the KSAs using the exact wording from the posting. (i.e., if a posting asks for experience with oral communication, say "developed skill in oral communication through giving powerpoint presentations to groups," not "gave powerpoint presentations to groups).
ETA: Also, the process is extremely slow, so it would be a good idea to get something else while you are working on this. Even if you are chosen for a particular posting, the process can take 6 months or longer. Civilian positions don't have housing allowances, and USAjobs postings reflect locality adjustments (unless the posting is for multiple locations, in which case it would be reflected in the range). You can look up GS pay levels by locality online.
I am just about to leave a federal job, actually. Mine is not a permanent position, so I didn't get it through USAjobs, but I have been around the process. A lot of (but not all!) positions are earmarked, so don't get discouraged about rejections. I have enjoyed working in the federal service, but I did get bored with my job which is why I am leaving. But, I am not opposed to returning to federal work at some point.