Optimistic locking may work for your application, but for some apps it's not an acceptable solution:

• When the user queries a row for editing generate a hash of said row
• When the user requests that the row is saved, regenerate the hash of the existing row. If it matches the original hash, then complete the save - otherwise let the user now that the row was changed by someone else

The problem that I've encountered with locking web apps, is that rows get checked out/locked, and then the user gets distracted, called into a meeting etc and they never remember to unlock the record. Optimistic locking solves this problem, but it may not suit your business rules