BESM taught me how to "pick" your ally. It's not just looking and judging and picking. You have to make them 'feel' like you've been picked. The personal touch always helps. I always make sure to have a 'diplomatic part', a 'personal touch' part and a 'strategic part' in all my press. Just enjoy the chit-chat between your fellow diplomats.
I used to play serious Diplomacy where I would try to analyze the other players and offer them strategies without thinking about whether they trust me or not. Don't even get me started about the time I lied to every other country because I made too many promises. I found that this does not work. Instead, just relax it.
If someone is a strategist then let him take the lead in that department. I love thinking of the perfect moves and offering to my allies. It's something I'm pretty good at too. If you do as they say and play along with their press then it turns out to be a fun game for them and for you. Remember to work with who you're dealing with. Remain the same person but don't say the same things when dealing with the different playing styles.
Like SpellofWheels said, "They are constantly talking and constantly offering ideas." Chat till their ears bleed. But don't come off as annoying, pesky, or even nosy. If there's something to talk about, talk about it. If there isn't, well, there's always something to talk about. There's one exception to this: when you're at war. I used to say to those I stabbed, "It's okay. We might be able to work together this year if you have any ideas on how that would work." I was afraid of being stabbed by my ally so I tried to keep all my options open. Yeah, that was daft of me. Instead, I found that my press with my enemy often led to my ally stabbing me! I often got wound up with promises I couldn't keep, and that's always a mess to clean up.
As for finishing the game for a win I've come up with only one way to do so--you have to get your armies ahead of the stalemate lines before you take the Supply Centers. It isn't so much progressive gain but the one big year where you're going to lock in the win. Either that, or just go big or go home and hope for the best.
This is my general assesment on how to play the game as I've learned lately. I can't say I'm all that good or am an expert on the game but I've done well lately. I hope this helps and I hope you already knew most of this stuff: it's pretty general.