As my regular readers will know I am smack in the middle of a 6 week hiatus from BJJ to heal up and get ready for Mundials training stretch, but I'm not the only one with some downtime. JiuJiu and Meg are both rehabbing injuries as well, and both of theirs are more serious than mine. So what the heck do you do with injury downtime? It's a common question that pops up at Jiujitsu Forums on a regular basis along with the more generic "I Can't train for a week/month/decade how do I keep improving?" questions. So here are a few things you can do to help you avoid backsliding too much during your downtime.
First of all don't neglect your fitness! Just because you can't do jiujitsu doesn't mean you have to turn into a couch potato. I picked up Yoga For Fighters and I've loved it. I also have free weights at home so I'm able to lift weights in ways that don't aggravate my injuries. JiuJiu has a pretty horrible sounding back injury and is still kicking ass and taking names on our Fitocracy (Invite code DACEU) leaderboard by doing an array of bodyweight exercises that aid in her recovery.
Secondly, READ. Pick up some books, Saulo Ribiero's Jiujitsu University is one of the most highly recommended books around and I throw my two cents in on that as well. Buy it, read it, re-read it, put it into practice. Another great book I recommend is Drill to Win by Kevin Howell which will lead you right into my third point.
Third on the list is solo and partner drilling. There are hundreds of great solo drills you can do to build agility and flow even if you can't do anything really strenuous, and even if you can't roll actively in class attending and drilling with a partner can be highly beneficial. Many people find their greatest improvements after periods where they couldn't roll for a while and spent more time drilling. Drilling is a fundamental plank of BJJ that is often overlooked because it's not as much fun as rolling. Forced down time is a good chance to work on that aspect of your training.
Fourth is to clean up your diet! You can always work on improving your diet. I picked up the Samurai Diet on the recommendation of a friend and it's a great book. Switching to Paleo or Primal is always a good idea, but even just reducing your sugar intake and resolving to shop the perimeter of the grocery store (Vegetables, Fruits Meats, and Dairy) instead of the inner aisles (Processed garbage!) will help you in the long run.
Fifth and final is to watch videos. Watch your old competition footage and wince at how awful you were and figure out where and how to fix your mistakes. Watch training DVDs. Watch competition footage from some of the best in the world over at BJJFights.
Staying mentally active with your jiujitsu and physically active any way you can will help you avoid backsliding and losing progress during your downtime. You're still going to lose some progress, but it won't be anywhere near like what would happen if you just sat around eating poptarts and drinking cherry coke for six weeks.