Retirees, what do you know now that you wish you knew then?
#1 If you have not already done it finish your degree at least Bachelor's, if you have a degree attempt to get a higher one, i.e. Masters, PH D, JD, etc., most GS positions of any value GS-9 and above are so competitive now because of the economy and cutbacks and all the unemployed veterans won't even register you as being qualified w/o a minimum of a Master's.
#2 Get as many certifications as you can for the field you have an interest in going into especially since the military will pay for most if it is associated with your current job, if it is IT get all and every one you can, most companies in the IT field won't care as much about your degree in IT as they will about the certifications as they cost a great deal of money to get them.
#3 Try and ensure your clearance is as current as possible and if you can swing it even get it upgraded as most GS or contract jobs will require an active or at least one that has a few years still left on it for transferability so it won't cost them the money to get it done. If you plan on getting an overseas job, it will need to be current and active, so try and line up the job so you can roll into it while on transition, that way there is no break and it can be rolled over with ease.
#4 Probably the most important one I wish I had known, if you don't have a guaranteed job lined up to move right into before retiring, don't go, since you can start the process at almost two years out, start your networking and getting everything set up and lined up about a year out, if you are unable to do that, you may want to consider staying a little longer and moving to a new or higher position if available. The current economy is not very friendly to those getting out without a plan and job ready to go, and even those easy contract jobs around every military base are starting to dry up and becoming overly competitive, something that was extremely rare 5 years ago as there was plenty to go around, not any more.
That is the best offer, I am sure some of my other retirees and veterans have similar if not more in depth answers as well.