When building your activity network, the following items are considered Best Practice...

PLEASE ADD TO, OR AMEND THIS LIST...

1. All tasks / activities have predecessor and successor
2. All tasks / activities have at least one Finish-to-Start successor
3. Resources are assigned to tasks / activities and not summary activities
4. Milestones do not have resources / budget (a milestone does not have scope)
5. Thou shall not have negative lag on a task / activity.  Most people are pretty soft on that, but general opinion is that negative lag predicts the future with certainty and that is poor logic
6. GoldyLocks was right - tasks are not too long or too short, they have to be "just right" in duration. You have to decide what that means and set sensible / justifiable durations. ( Quantiies & Norms will help you achieve the correct duration and the addition of available personnel to complete the task will help finalise the duration)
7. Too much positive lag on a task probably indicates missing tasks
8. Constraint types: You want them to be As Soon As Possible as much as you can. Start-No-Earlier-Than, Finish-As-Late-As-Possible, and Finish-No-Earlier-Than are tolerable. Items like Must-Finish-On, Must-Start-On, Finish-No-Later-Than, Start-No-Later-Than are pretty much not a good thing.
9. Constraints in the programme should not limit, or affect, the CPM analysis of the network
10. Check your logic
11. Avoid the open ends in the programe
12. Every activity gets a baseline