As I've only levelled an Inquisitor past 20, my experiences will be confined to that.
I milk conversation gain like crazy - I spacebar/esc to try and find the optimal conversation path to get me max affection while still getting me only DS points.
I've noticed a trend - the planet you acquire a companion will have zero to near-zero affection gains from conversations - besides, of course, the quests that specifically involve them. I tried to take Andronikos Revel around Tattooine, every dialog I did wouldn't please him at all, so each time I'd give up the conversation, drag Khem out, and do it with him, getting Khem affection. I'd begun to think he was bugged. However, as soon as I hit Alderaan, bam, conversations give rep with him.
Same thing happened again with Ashara on Taris, nothing from conversations. It also seemed this applied to Quesh, but that's a miniature planet anyway. As soon as I hit Hoth, though, there's plenty of affection to be gained from conversation.
I suspect (though I have not confirmed) that they didn't program in affection gains on the planet where you first acquire a companion. When you get to Hoth, you'll be rolling in Ashara affection, I promise (assuming you do things that please her).
As for gifts, there are three categories to affection gain for those like (slight gain), love (moderate gain), favorite (large gain). Ashara has no love/favorite gifts, she's only got likes. It sucks.