To scale well, a multi user application has to not use any locks or to keep them as narrow as possible.One fine approach is to develop some kind of message queue system to manage request/response (no locks at all). That will allow to make partially offline system too.

Particular solution depends on RDBMS system U have. In MsSQL U can do it manually or using MSMQ (or other ready but not cheap solutions like Biztalk etc).

Best regards,
Krzemo.