Three ideas:
- Make them all access free for the first week they're up, then charge for access to the back catalog. See CreativeLive[1] for a good example. This way everyone can measure the quality of video, audio, transcripts, etc so they know what they're getting, and the people who need to learn a specific topic will have a strong incentive to pay. This will encourage you to cover evergreen topics to make your back catalog more valuable
- Flip it and make the new videos available to subscribers for the 1st N months, then free afterwards. This will bias you towards covering the newest, churniest stuff that is causing people pain right now
- Do like RailsCasts and alternate between free and paid videos. Paid subscribers get twice as many videos, everybody still gets something free for marketing etc
Personally I think the first one is your best bet.