In order of effectiveness:

1) 02 injection

2) Cooling water with Ice - Calms the fish and cool water absorbs O2 more readily.

3) Livewell additive - tried some Tournament Ice the other day. Worked good, but haven't made up my mind 100% yet.

4) Fizzing or weighting fish caught deep will help as well.

In a perfect world, you'd have 02 injected into a livewell that is on recirc with no new water coming in and drop ice bottles in there occassionally to lower the temp by about 10 degrees. Add 1/3 to 1/2 fresh water every 3 hours since amonia will build as the fish breath and waste is deposited. In addition, add an additive of your choice. If your fish are coming out of deep water, then fizzing their air bladder will help them survive or weighting their anal fin with clip on weights to keep them upright until their air bladder deflates.

Wannabe...