good thread topic
i posted something like this for a noob that was having a hard time but i think you hit all the point i did so i won't try to track it down and repost it.
2) warships- transports seem to be MUCH more irritating than attack ships. if you buy transports from opponents make sure to get them out of his territory ASAP. getting them beyond his influence borders is NOT enough. make sure to keep them at least one turn from the border, minimum.
3) flipped planets- often a flipped planet will come with a nearly completed starship. turn this into a transport before you turn it back over and run off with some of his ppl. this will also make it flip back to you faster. think of it as removing those annoying dissedents for him so he doesn't have to

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4) trade routes- don't under estimate the effect of this. make sure you send most of your trade to the ones that are most dangerous to you. check your diplomacy report and find the guy giving you the the most -effects and throw him extra traderoutes(unless he is too wimpy to bother with).
5) wonders- also build galactic bazaar(master trade). this will magnify the effect of your trade(+50%), and gifts ARE trade. i am not really sure how much they value gifts, but unless they are all concidered equal this should help. grab Restaurant wonder too, but not necessairly at the expense of the diplomacy ones, they take priority.
ah, i see one you missed, sorta. starbases.
9) influence and military starbases in their territory cause conciderable friction. don't build these until you are prepared to fight. economy starbases are fine tho so you can pump up your trade routes if you like. go ahead and do mining starbases too. these make tempting targets so reinforce them, but worth the risk.