Now THIS is the kind of anonymous question I like. Thanks for asking! And congrats on graduating 101!
Here are some suggestions...
The first is keep improv fun. As you get into Level 201 and beyond it can get more heady and difficult, but remember that it's supposed to be fun and creative above all else.
Don't let it take over your life. This is tough, too, because a lot of your friends will take class after class after class and you might feel you've been left behind unless you put down just as much money and time. You won't. Improv will always be there.
For practice groups...
Here are two IRC links on the subject, some of which might have repeated advice: "Where do you start?" and "Tips on starting your own group?"
Starting a practice group is mostly organizing the time/space. I'd talk to your two or three closest friends who are dedicated to practicing and figure out a time that works for you.
Book that time (two hours or three) at a rehearsal studio: There's Simple Studios, Pearl Studios, Champions Studios, 440 Studios, Studio 353, just to name a few (google 'em).
Then ask around until you have a number of interested people. I'd suggest quality over quantity. Ask people you want to play with; don't just settle for bodies to beef up the numbers and lower the price.
Of the people you ask (say, 10), maybe six will be able to do that time, and one or two of them won't show up. There's your potential practice group!
My personal preference is to cap a practice group at eight people, so everyone is always up.
Hire a coach. (See below.)
After a practice or two, if everyone wants to meet regularly, set up a kitty for the money. Basically, figure out the total cost for four rehearsals (space + coach) and everyone owes their share. Now if someone flakes out, the people who show up to rehearse don't have to pay more (a pet peeve of mine).
If people don't want to meet regularly, or if it peters out eventually, don't sweat. Practice groups come and go.
Book more spaces for the month. Book more coaches. It'll make things easier on you if you can dole these responsibilities out to other members of the group. One person shouldn't have to do everything. That leads to burn-out and resentment.
Now, when it comes to coaches...
I would definitely NOT recommend going coachless, and not just for the reasons you mentioned. Having a coach forces you to be more serious about the practice and the scenes. (If you just wanna meet and do warm-ups, though, that's a different story.)
Try several different coaches until you find one that fits your group's style and aesthetic. Seriously. Give yourself at least a month to test out different people.
Don't just go for the veterans/heavy hitters! Especially since your group will be made up of newer improvisers, a newer coach would have more availability, would probably be cheaper, and I think they're often more dedicated. Test 'em out.
Make sure the coach fits your time, don't stress about adjusting your time to the coach. They're the most replaceable person in your group.
Here are links to coaching threads on the IRC and on UCBcomedy, and I'm always happy to recommend people through email.
Hope this helps! Anyone else have advice?