Tip #1. Join your territories first, then go for additional territories. A common mistake is to go for the enemies large numbered territories first, before joining yourself. When you lose against a large number before joining, if may have cost you the game, especially on your first few turns. The only times you may consider not doing this is if a) you have lots of territory already b) you are in a dire situation, where even joining wont save you. In which case go for the large number as long as you can still join after the win c) your attacking piece far out-weights the opponents number.
Tip #2. Never trust an opponent. That is an opponent who is your ally or otherwise. Don't trust them to do the next logical move either. There are plenty of players that will play illogically!
Tip #3. Use your enemy to fight other enemies. Sometimes you are presented with a situation where "opening" an opponent's trapped large territory is a good idea. If your main territory mass is far away enough from the large enemy(1) territory, and you need another enemy(2) to be mitigated that is adjacent to the large enemy(1) territory, then open it up on your next turn and watch enemy(1) kill your other enemy(2) for you.

Tip #4. In regards to HardCore on Random settings, watch your expansion rate. HC is all about having high numbers in your territories. Spreading out too much may cost you more than you make, especially if it opens you up to being split. Use caution!
Tip #5. Use your boosts at the beginning of the game. Not all players subscribe to this tactic, but I have the best luck of ensuring a win by deploying my attack boost mostly on the first turn. By being aggressive and grabbing as much territory to start while the numbers are fairly low, I help to ensure that a) my enemies starts are rougher b) my leads start off more commanding. Generally, I will not use all my boosts, as save a few for later, but normally I burn through all but 2-3 of them. I also usually use them even when I have +1 more men in my attacking territory vs the opponents. Again, I want to ensure a strong and solid start. If not for a good start, you boosts are often useless later on, as it is too late to regain the traction and territory you need.
Tip #6. Don't pick on everyone at once! While I do not like alliances, being targeted in your attacks over a few turns is not a bad idea. Getting everyone's attention by attacking them all is often not the best way to start the game. So pick and chose wisely. But when the time is right, strike the others in coordinated, timely, bulk attacks. This said, you may get unwanted attention of someone by attacking them too much. Feeling singled out by you, the opponent may just target you for the rest of the game... so don't be over-bearing. You have to feel it out... sometimes a nibble from each is appropriate.
Tip #7. Using misdirection in chat can sometimes help you. Acting small and playing big is a good tactic. Pointing out how big someone else is, may take the heat off you. I can't tell you how many times I have waited to "look" small, while having big numbers waiting. While I look small (say a minor split or two), I randomly state in chat how big another opponent is, and that the game looks over. Next thing you know, the apparent big opponent is attacked by many, and come my turn, I take the advantage in a big way. Timing this is crucial, and this can work miracles.
Tip #8. When all else fails... ragequit. The game leaders will usually appreciate you that much more.
