I know I'm not the most skilled player here, but I thought my advice could still be useful, and maybe I could learn something. This is what I'd suggest for any player.
Talk to all sides from the beginning, using a logical reason to support you even if it sounds stupid to you. The more you talk the more likely they will join you especially if they're inexperienced. Try to stay neutral (I always have trouble with this).
Use the beginning phase of the game to test the skills of your enemies, tactics and diplomacy. If your enemies are inexperienced, use your tactics and manipulation (convince each power that you will support them against each other) to overpower them quickly. If they are experienced, find someone at about your skill level and ally with them for most of the game. Plan out how you think the game will go in your head, and whether you might need to betray your ally.
If I'm facing a skilled player, I come up with the most basic, and safest strategy to advance. Then I try to imaging what my enemy would do, and I counter it even if it means taking risks. You learn a lot this way.
Diplomatically, you should always be analyzing the motivations of the other players. Who is likely to betray and how can you take advantage of it. By encouraging the two players against each other of course. If you think your ally is going to betray you then tell them your suspicions. Do everything that you can to counter them without threatening them. If you don't want them as an ally anymore, then betray them preemptively, at the right time (I sometimes have a problem with this).
If you want to create an ally from an enemy, talk to an enemy that would likely work with you. Their position in relation to their ally, how much you will support them, the difference in their SCs, and whether their a central or edge power have a lot to do with this. Use logic and reason to tell them all of this.
Other useful advice:
Depending on the skill of my enemy, if I have three units attacking, and one to move, I usually choose the one that is most likely to be attacked. This way your supports aren't cut, and you don't waste time. Then ideally I have another unit backing it up to bounce out the enemy.
It can also be beneficial to leave a territory and retake it in the fall, when it is actually important to hold the territory.
Sometimes there are if your in a desperate situation and your enemy is overpowering you, using support holds is not the only defense. If you have enough units, you can cut supports back up by two or three units supporting a move towards the unit that is moving by itself.
One of the most useful tactics in gaining trust or an ally is giving up an SC, or trading SCs. This is especially true if the SC your giving up can't be defended anyway.
Never give up. I've played games against players who reached 16 or 17 SCs (they were 13 or 14 when I allied against them), but I allied up with two or three other players and we pushed them back to a draw or so that someone else could win.