I see the lover's actual deaths as reactive. I think this shows despair rather than love (although one could argue that the despair is evident of how deeply they did love). Still, I don't see the acts themselves as evidence of love.
Here's my three:
The nurse proves her love by helping Juliet even though doing so is against her employer's best interests. She risks her job to help Juliet. She also proves her love by suggesting that Juliet marry Paris instead. Once she knows events won't work out as expected, she tries to get Juliet well placed--and fast.
The friar also proves his love by going against the interests of his employer (so to speak) when he marries the two lovers without parental consent or the saying of the banns and again when he gives Juliet the potion that gives her the appearance of being dead. Not only does he place other considerations over the laws of the church here (he acts within the dictates of his conscience when he says that the marriage might end the feud), but in dealing in drugs he also meddles in God's affairs.
Finally, Juliet is willing to risk death to keep her body and her love pure for Romeo. In taking the potion she knows she might not survive.
All of these involve a conscious decision and a willingness to place the best interests of others above their own worldly concerns.