One of the best things to do is seal penetrations into your attic and put as much blown in insulation there as possible.
That is what I did, more blown insulation.
Here in Tucson we need to keep the house from heating up so I have been working on insulating my place like a thermos bottle, new doors are next but since I have to make them (that is my business after all) they will also be last.
So far I have:
Replace the roof with heavier grade architectural shingles but passed on the radiant barrier underlayment, the cost/gain ratio was not worth it for my small house (750 sq. ft.). The small house was also planned to lower my overall energy usage by not heating and cooling space I never use.
A new 14 SEER Tempstar gas pack sits on top of the roof.
Single glazed steel casement windows were replaced with Marvin Integrity with Low E3-366 glass, the West facing windows have practically zero heat gain, amazing stuff.
The house was outsulated with 1" high density foam and stuccoed with a marble dust plaster with integrated color so I never need to paint, the 1" foam keeps direct sun from heating up the concrete block so there is less indoor radiant heat.
The attic has R30 blown insulation on top of the existing R19 for a total of R49.
During the summer I turn the thermostat up to 84 degrees (from 78) when I leave in the morning, when I return at noon the inside temp is still below 84 and the A/C never went on.
I paid between $32-$41/month for electricity between Oct. and May (8 months)
June, July, August, and Sept. electric costs were between $82 and $92/month.
I have ceiling fans in every room and the house it very comfortable year round.
Wayne