I'm new to ACT!  These tips would have helped me a lot.

• Figure out how you want to measure your performance, and the performance of initiatives you take. 
• The core of the program is Opportunities
• Create an opportunity for every prospect contact you enter.
• User-defined fields in Opportunities are more useful than user-defined fields in contacts 
• Make your own sales process, with your own stages
• Each stage should represent what the prospect is doing, not what you're doing. For example, if you send samples, you should have a stage called "Evaluating Product" or "In discussion" or "Unresponsive." If you have a stage called "Sent Sample" you won't know what's going on with your customer and you won't know what to do next.
• I have a stage called Lost. To me, it makes sense as part of the stage progression.  I double-enter it as Opportunity Status,also.
• Send all your e-mails through ACT!  That's the only way to get the whole history in ACT!
• Every time you receive an e-mail from a contact, add it to their history manually (Attach to Contact).  Incoming messages from customers MUST BE ADDED TO THEIR HISTORY MANUALLY.  Messages you send will go there automatically.
• You can't access the SQL tables via ODBC, so you're stuck with their queries (which are a little esoteric).  This is a major weakness for reporting.  You can't group contacts and compare progress between groups (was I more successful at the Vegas show or the Atlanta show?).  You can't connect group to contact and contact to opportunity stage and use it as the basis of a single report.  At least I haven't figured out how, without a real query engine.
• The ID/Status field in the contact record is for high-level, slow-changing classification only, i.e. friend, vendor, coworker, prospect, customer, etc.  You won't be able to find out opportunity stage information broken out by that Status.  For example, if you classify "cold lead" and "hot lead" and later want to compare your close rate between those two, you can't.  Same with contact user-defined fields.  To get this kind of selling performance data, you have to stay inside the Opportunities fields. That's where to keep customer progress information that changes through the sales cycle.
• I have no idea what a group opportunity is.
• Always update the Last Results field for a contact.  It's the best place to put that one-line description of their situation.
• Every time you interact with a customer, make sure that you:
  • update the Last Results field
  • add (or check) the Activity records (what's your scheduled next step)
  • update the stage of your sales process they're in
• Before you open a closed opportunity, note the date you closed it.  It's cleared if you re-open it (which you must do to correct info, for example).  So 1) note the date, 2) re-open it, 3) correct whatever, 4) close it 5) re-enter the date.

That's all for now.  I'd really appreciate it if someone shared their own do's and dont's.