smah was too nice. Get a router. A dsl router from a good SOHO company like d-Link, SMC, Linksys, etc. is now available after rebate for between $16 -$49.95 at your local computer store, or from on-line sites like Amazon.
Leave the modem and router on 24/7. Given current pricing for a dsl/cable enabled routers, it would be silly to suggest to you any other alternive way to share internet access.
Do not consider alternatives, use a hardware router.