Use a wye fitting rather than a tee so that less suck is required to move dust.
Clear ducting for dust collection system. - brilliant
Shrink PVC pipe for your ductwork needs
Rather than a short radius 90 degree turn, incorporate two 45 degree bends with a section of straight pipe in the middle, which will allow the particles to scream right through the turn.
Dust Collection Plans - by David Grimes @ LumberJocks.com ~ woodworking community
Downdraft Sanding Table Box - Learn how to make a simple box that will give you excellent dust collection for sanding your woodworking projects. FixThisBuildThat.com
Central Dust Collection - Dust Collection Tips, Jigs and Fixtures | WoodArchivist.com
Miter Saw Dust Collection Hood - Miter Saw Tips, Jigs and Fixtures | WoodArchivist.com
Adding A Pre-Separator To Your Dust Collection System
Need Design for Drop-Down Dust Collection for Unisaw