What to do if...

[Fishin]

Something that makes a good lead is usually a good choice. It's generally a good idea to choose something that can counter pokemon your opponent might send out based on what you've seen of their team is good as well, obviously.

 

[Sam I Am]

Depends on the pokemon I just eliminated. For instance, if I just took out a Blissey then I'd send out a special sweeper like Starmie with Life Orb since I know their special wall is gone and Starmie has nice type coverage.

On my current team though, I usually send out Heatran since mine has a Choice Band. I used to switch to Scizor to hopefully get a free Agility and SD in to BP to Breloom.