Three General Principals in order of bang for the buck.
1) Maximize the “Air Pump” volumes - i.e. flow head, match ports, improve exhaust.
2) Reduce internal friction – i.e. roller rockers, blueprinting tolerances.
3) Lighten rotational mass – lighten internal engine components (best left to the pros)
Personally I’m not a fan of “big” cams on a street car – that said there are some great cams out there for street use (here is a nice overview) - http://www.compcams.com/Pages/419/camshaft-breakdown.aspx
Only those who will risk going too far can possibly find out how far one can go. ~ T.S. Elliot