1) Through genealogy group mailer, PSAs mailer and CSAs mailer.
2) You can stay in contact with them also by their mobile phone, skype, YM or any chatting media.
3) If your CSAs and PSAs are within your locality, You can invite them for a small gathering as sort of bonding.
In own experience I'll contact them at least weekly or just send them genealogy group mailer every week. Separate the message of your Inactive, your movers, and your new CSAs ands PSAs.