I fly my -7A into the DC SFRA on a monthly basis to Tipton(FME) and/or Freeway(W00). Recently, I've entered from the south over BRV, but have entered from north over FDK. I second Gaithersburg(GAI) as a good choice.
FME is also a good choice. It's better for transients than GAI, because they have dedicated transient tiedowns and GAI does not. You can get an Enterprise rental car at FME, if needed. Lately, I've been using W00, but transient tiedowns can be iffy, without an advance call.
Because of the strict transponder usage rules, i.e squawking 1200 is forbidden and an automatic violation, I leave my transponder on all the time. When inbound after reporting your landing airport "in sight", ATC will remind you to "stay on your current transponder code until you land."
After contacting ATC with a request to pick up your SFRA flight plan into "XXX", they will give you your transponder code. The next transmission from ATC, which is your approval, initially took me by surprise, because you're NOT cleared to do anything. ATC will say something like,"Transponder observed. Remain clear of the Class B." That's your approval to proceed to you destination via your flight planned route, but stay out of Class B airspace.
Typically, that's the last you'll hear from ATC, 'til they hand you off to the next controller or you call your destination in sight. If I sense that ATC isn't too busy, I'll request clearance through the Class B at an appropriate VFR altitude, usually 3500 or 4500. I'll get it 90% of the time. That avoids having to get down below 1500 for 20 or 30 miles.
It really helps to have a very specific route programmed into your gps ahead of time. For example, I arrive over Nottingham(OTT) going to FME or W00. A direct route to either airport from OTT sends you right through the Andrews Class D (7 DME ring), so I programmed an 8 DME arc into the gps and let the autopilot fly it to keep me clear.
Departures just require a quick phone call to Potomac Approach to get your code. Just make darn sure that the code is in your transponder, which had better be on when you call ATC after departure. Again, that's why I leave it on all the time.
Anyway, it's not difficult, just a few specific rules to follow. In 5 years of doing it, I haven't had any nasty calls from the FAA. I just love the thought that I'm poking the bear by flying in. The feds seem to make it pain just to keep people out, so do your homework and fly on in.
Now, if you really want to make it stress free, file and fly in on an IFR clearance, even in VFR conditions. You'll probably get a little more convoluted route, BUT it will be like a red carpet has been rolled out for you. One time, I came in right over BWI at 2500 inbound to FME on an approach. ATC will likely vector you at a reasonable altitude and all of the Class B avoidance issues disappear. Just don't squawk 1200 after you cancel.
Mike