Preventing router tear-out on cedar

Be more careful.

Seriously, you nee to provide way more details to get any reasonable answer.

Are you talking finger-jointed or something else cut by jig? I've always found that using a fresh backer board works with any sort of wood with through cuts. Depending on the look you are going for, knife-scoring at the deepest point of the through-cut will minimize tearout (at the cost of a scored line showing up, of course)

Backup the area that you are coming out of with the cutting edge with a piece of wood clamped the to edge.

If routing a groove, use a down cut spiral.

While forming an edge, I had a large section blow out in the middle. I glued it back in, then raised the bit for the remaining cuts, then lowered the bit for a final cut, that way I would not ruin any other parts. I used sawdust as filler on the piece glued in. I also moved a little slower, but not too slow to burn the wood.

Tape does help.