In my community we've got a variety of different re-spawn rule sets for different turnouts, game styles, and field sizes.
The most basic is the static set-time re-spawn. The spawn time is announced before the game starts, and depending on the field size and amount of people that showed up, a 1-5min re-spawn is used. A cheap easy way to be sure everyone spawns on time is to buy a couple big, battery powered analog clocks and put them up at the teams spawn locations.
For mobile re-spawns/FOB's (in certain game types and again depending on the turn out) we use a 3-5 man touch-and-go re-spawn.. Meaning you set a specific minimum number of people, lets say 3, and you have to have at least 3 people touching the flag at the mobile re-spawn in order for them to spawn back in. For larger events this can be coupled with a standard time re-spawn and the mobile re-spawns need to be set-up by the players and can be destroyed by opposing players, adding another strategical element to the game.
My favorite is the 5-minute-all-in rule. Re-spawns are allowed every 5 minutes on the analog clock and everyone waiting is back in at the same time. This means that if you die and get back to re-spawn at 12:04:30, you've got a 30 second re-spawn. But, if you get back at lets say 12:00:30 then you have to wait 4 1/2 minutes. We play primarily mil-sim games and this helps a lot to keep squads together on the field.
I'm on the same boat as you as far as medics go. It's really frustrating to see people you just blew away back in the fight in >30 seconds in some cases. Our community does utilize medics with pretty standard rules (limb hits mean you bleed out in 2 mins unless a medic can tie a bandage to your arm, after that the next hit you have to go back to re-spawn.) I actually thought of this idea as I was typing this out, but I'm going to propose a new rule to our community and feel free to use it. Medics must move wounded out of sight or behind cover before the injured can get back in the fight. Of course I don't think this would work on every airsoft field out there, but it wouldn't hurt to try.