1) If you are a CAAP/GA client, talk to your worker. This is the easiest way and they can authorize you for as much shelter time as you need as long as you are in good standing with the shelter and CAAP programs.
2) For anyone: go to a *city resource center* for a 1-night or 7-night reservation. Resource Centers include Mission Neighborhood Resource Center (165 Capp St), MSC South (525 5th St), Tenderloin Health (187 Golden Gate), Glide (330 Ellis), 150 Otis (150 Otis St), etc.
3) Next Door shelter has a waiting list for 6 month case management beds. You can sign up for this waiting list there in person at 1001 Polk St (@ Geary).
Please do not show up at the shelters without a reservation. You will be directed back to a Resource Center.
This question was asked on the wiki: San Francisco Homeless Resource.