Use tokens issued for a user and IP combination. Use HTTPS to carry the auth for the user and return a token. Use the token with HTTPS or HTTP to continue talking to the server. Expire tokens after some reasonable time. Expired tokens indicate you need to reauth. If you lose a token, it could only be used by someone pretending to be you (and with your IP), so limit what a token can do via the service's API if you want.

Additionally, keeping a user's clear text password in a local datastore is usually a 'bad idea'. At least store the MD5 or SHA-2 of the password (MD5 is considered insecure now though) and then pass the hash along to auth the user before giving them a token.