Proper multi-users design

Hello,

I've inherited a single user application using a database (multiple backends : Postgres, Firebird, Oracle) and need to bring it on par for multi-users access.

Right now each DB operation (fetch a structure, update a record, ...) is embedded in its own transaction, and I'm not really sure where to begin.

I'm not really looking for a specific DBMS procedure (that I can figure out myself), but more for a generic best practices/how-to implementing proper multi-users access.

I've searched a bit, but apart from the generic optimistic/pessim. locking mechanisms available and transaction isolation did not find anything worthwhile. Is my best bet to rework everything to _really_ use transactions ?

Do you have any links, articles or books to recommend ?

Thanks in advance,

--
Ael