HRE

[Martok]

Best bet to avoid an early war is to build a paper tiger army of urban militia. They can't fight very well but it will look impressive.

Yeah, I've employed that tactic before with some success. It's not foolproof, of course, but it does seem to discourage other factions from attacking you.

[gollum]

Usually its best to take out the Italians quickly... this solves quite a few problems and makes the Pope more of a pain, but a harmless one. Afterwards certain French regions in the west can be easily taken as Ile-de-France, Flanders and Champagne, this usually quiets the French permanently. Taking over the Poles solves a frontier problem and silences a permanent pest, while taking over the Huns is the most ludicrous and promising, but usually at the same time one may be fighting a Spanish or Almohad invasion from the Iberian. The Danes are a toothless tiger because they have to attack accross the bridge to Saxony. Not worth the time to take out unless one has taken out all others already and has safe rigid frontiers and a solid healthy economy.

In many ways its interesting with the HRE because the direction of expansion/defence depends on the balance of power in other places of the board. The game can have lots of ups and downs that are hard to predict.

[I of the Storm]

After several campaigns (huge units setting, normal difficulty), I advocate a semi-blitz strategy. The best approach for success in an early campaign was to assemble all your troops in Franconia and Bohemia and hit the Polish asap. You should be able to take them out with your starting troops within 2 turns, while they - most probably - didn't expand yet. 1 faction out. Build towers, basic farming and forts everywhere else. Keep strong garrisons in Burgundy and Lorraine (important!), Austria and Poland. A small garrison in Denmark might also be useful.
Turtling, farming and teching is important now. Make use of any opportunities but otherwise lay low. In my game, the French were overrunning my allies the English, so I was forced to intervene on their behalf and secure Flanders, Champagne, Ile de France. That usually cripples them. Otherwise I only counterattacked. I took over northern Italy when they died out, so I took over. In 1240ish, Hungary finally attacked, and I'm currently finishing them off.
The Turks have recently been killed by the Byz, but everybody else is doing fine so far, which is good.

Taking out Poland early gets one risk factor of your back, and leaves some rebel provinces in eastern europe that you can take your time with. Plus their provinces aren't exactly poor.

[gollum]

Never tried this route before I, i'll doso next time i give the Kaisers a go. Seems interesting.

[I of the Storm]

I've been toying around some more and I've come to the conclusion that taking out Poland directly followed by taking out Hungary (yes, immediately after!) is a very beneficial strategy for the early HRE. With both kingdoms you share rather long, underdeveloped and not very defensible borders, in opposition to your other borders (Danes: river; French: 2 rivers+woods; Italians: mountains). Both kingdoms are rather weak at the beginning (Poland 2 provinces, Hungary 3) and prone to attack you sooner or later with hordes of cheap missile cav/javelin inf.
So right after finishing the Polish I recruited one woodsmen unit there and split my troops into three armies according to the hungarian strength. Simultaneously invade Croatia, Hungary and Carpathia and you're done. You can then either consolidate there and go bully someone else around (Danes, Italians) or consolidate there and take over the east which is what I did. I'm currently holding an eastern border at Croatia, Hungary, Wallachia, Moldavia (Avar Nobles, yumyum), Kiev (Steppe cav, spam'em) and Lithuania. The Byzantines, who I'm bordering now, are busy fighting the muslims, the Danes are ... I have no idea what they're doing. Fishing probably. I only have a peasant in Saxony and they don't care. The French are crusading, thanks to a little modding that gave them a chapter house and a crusade marker in the startpos file. Highly recommended little tweak: They are actually going after their GA goals, and don't overrun the english or your western borders with UM/archer hordes.
I'm pursuing the Empire goals in Italy right now which will get me excommed. We'll see how this turns out.

Edit: In case you're wondering what to do about the western border in the meantime: queue up spears, archers and UMs in Burgundy, Switzerland and Swabia and garrison Burgundy and Lorraine, that should deter the French as long as you need to build up your income, recruit more garrisons, deter them even more. You get the idea.