You can flush it yourself if you want. As long as your compressor is in good condition. What you have to do is disconnect the hoses at the condenser, evaporator, accumulator and back manifold of the compressor. There is a special tool to loosen the spring lock fittings to unhook the hoses. They sell them for around $10 at auto parts stores.
If you're going to do this on the car, you'll need either a compressed air, or a special pump to blow out the parts. If you remove the parts from the car, you can get by with lung power and some hose, or an air pump (like for air matresses, etc.)
Starting with the condenser (cooler in front of the radiator), fil it with some flushing solution and push it in with air.. let sit for a few minutes and push it a little bit more. Repeat until it starts draining. Catch the outflow in a suitable container. Then blow it out, and repeat until the flushing fluid comes out clean. Then blow it out good and let dry for a few hours.
For the hoses, fill them mostly with flusing solution (brake parts cleaner works good), let sit, blow out. If you catch and filter the outflow, you can probably reuse the fluid a couple of times. The suction hose (the hose which goes from the accumulator to the compressor) should be filled from the compressor manifold and pushed out the accumulator end. The high pressure hose likewise should be filled from the manifold and blown out the condenser end.
Note: The liquid line should be filled from the evaporator end and blown out through the condenser end. This is because you have a fixed orifice tube filter on the condenser side.. you want to push the contaminants back out.. not forwards.
Be sure to let the lines dry for a while to remove all the flushing solution.
On the evaporator (the part inside the car with the 2 tubes sticking out the firewall), push the solution into the top fitting (the inlet) and push out the lower tube (the outlet) which hooks into the accumulator (black steel canister with a wiring connector). You will need to remove the accumulator to get the fitting opened. Its a bit of a pain, but doable.
Now, as for the accumulator, I'd advise you replace it (around $60), but if you decide not to, then cap it immediately to keep it dry as possible, turn it upside down and let it sit for a while, then drain the oil out from the front facing tube. Don't flush it! The solvent could cause the dessicant bag inside to break and that'll ruin your system. Measure how much oil you recover.
As for the compressor, don't flush it either.. turn it by hand for a while until no more oil comes out. Likewise measure how much oil you recover.
Refill the accumulator and compressor with the same amount of PAG/POE as came out of them. Then add enough PAG/POE to bring the whole amount to 10oz. Including what you put in the compressor+accumulator. Spread it out between the condenser, evaporator, accumulator and compressor (1,2,3,2 if you needed 8) and install NEW Orings and lubricate with nylog (what the pros use) or mineral oil (yes even for 134a) on the tubes.
Now, snap it all back together, and bolt the manifold back on the compressor and intall the required r134a fittings.
Promptly take it to have it vaccuumed out and charged with refrigerant.
Also, check out this web site:
www.ackits.com/mastercool/flush.htm
lot of good info there, and they sell just about every thing, including a flushing kit.
Now, if all this sounds like too much, call around and get a explanation of the work and prices from a few shops, ask a few questions about the procedure they will perform, and take it in. They have equipment to do the job a lot faster than us.
Hope this helps.