I did a lot of military bases in the mid 90s, mostly in Germany, through (If I remember correctly) Satellite Entertainments, a guy named John Metcalf (again IIRC) working out of Tungelroy in the Netherlands. After a while, on the back of the German gigs we ended up doing S.H.A.P.E in Belgium, and a number of gigs over here (RAF Waddington, RAF Linton On Ouse, Catterick, Hedley Court, Edinburgh, and a number of locations around Aldershot). For the German bases we always seemed to be on with other acts from the same agency, even the DJ, so I assume a handful of agencies had it pretty much sewn up at the time. The British gigs came off the back of the German ones, People moving between bases, copping for putting on entertainment and remembering us (it's a big deal to pull of a great event for those given the task, and they're very grateful when you make that easy for them), and also recommendations between related units.
Security was fairly heavy at first, but with the regular gigs once we were known, they know the van, they know what time you're coming, they know who's in the van, etc, we were usually through in 10 minutes. We were always really well looked after, fed, watered, accommodated if required, and had some fantastic times. Senior Ranks gigs can be a little stiff early on, junior ranks stuff is just carnage. A professional attitude will get you far, the guys tasked with putting the gig on have a lot of reputation riding on the outcome, and if you make it easier for them to pull off, they will go out of their way to do you favours. For example, as a result of going out of our way to help on a number of occasions and putting on a great show, we were adopted as the 'house band' for 17F Squadron (Black Knights) on RAF Brugen, they would book us for anything, and if they knew we were in the country with a day off they'd set something up. They'd take us down on the Squadron to see the Tornados, private parties for send offs, etc.
If you get chance to get your foot in the door, do it. Go in and do your very best job, follow any rules you're given (you won't take any flak for misbehaving, but some poor bloke who's already got a lot on his plate will), and they will remember you for it.
Also, you will never buy a drink... the notion that you would pay for a beer seemed almost offensive to them!