Also, build a culture center or two, or three... to spread your influence and slow or counter their's.
If you can afford it, build a trade center or two...to build a chain of trade ships to boost income.
Also, consider more capitals, they cost more, and of course are more horrible to lose, but they hold their own pretty well and boost culture and have a range of abilities the frigates, for all their cost-effectiveness, dont.
Play closer to Home, dont expand too fast or get caught with small fleets out on the rim somewhere.
Expand slowly and build up as much as possible. Planet defenses are not a bad investment, they boost ship performance and add protection and dont leave your planet defenseless if you dont have a fleet there.
Its better to defend one planet with a good sized fleet and re-inforced planet, than two or more with smaller fleets and less planet defense.
Just some ideas,