I will ignore gunboat because holy fuck if you aren't the first one eliminated as Austria.
Tips in standard game:
1. Talk to Italy. Get Italy to commit to attacking Turkey. Turkey's success is directly correlated to the destruction of Austria and then Italy. IA are natural allies against Turkey. Sell anything you have to. Promise 2 of Turkey's centers as a starting point and offer 3, explaining you're content with the Balkans, if 2 isn't enough for your Italy.
2. Establish a friendship with Germany. Germany knows its game is in the West for the next few turns. A strong Austria generally prevents a strong attack coming in from the back. An antagonistic Russia can be subdued by the threat of a bounce in Sweden in the Fall of 1901.
3. Turkey is your enemy. Russia can be a potential ally. Doesn't matter. Make them both think you want to work with them. Get one of them into the Black Sea. A bounce can be worked out between the two of them and you're looking down the barrel of a juggernaut. You will lose that game. A successful move to the Black Sea is antagonistic, even when agreed upon, and it can make the juggernaut nearly impossible to pull off or tenuous at the very least.
4. Take Serbia and take Greece. You can make a stab at Romania or Bulgaria (with Russian support). However, neither of those are guaranteed. Serbia and Greece are yours if you want them and you haven't absolutely fucked up your diplomatic relations with everyone. Your third army should almost always bounce in Galicia.
Situation specifics:
1. Italy attacks
Either you have a novice Italy or you did a bad job and let someone else turn Italy's head. Damage control time. First, do not abandon your plans for Serbia and Greece. Gaining 2 centers and losing 1 in a strong position is better than gaining 1 in a weak position. if Italy is not yet in an Italian center, sell him on taking a shot at Germany or reinforce the benefits of the Lepanto. Do everything you can to make sure that Tyrollean army isn't heading south. If you bounced in Galicia, utilize that army in whatever way best guarantees 5 centers. If Italy is already in Trieste, it's probably time to see him on the Blue Lepanto. You're going to want to use all the tricks you have up your sleeve to grab Bulgaria (as a first choice) or Romania (if you just can't get into Bulgaria. In this scenario, Italy is putting his already gained center at risk, so you need to work on him a lot. You get Serbia back next turn, and you end up in the position you wanted with an Italian army already on the doors of Constantinople. If you wake up and it's 1902 and Italy is hanging out in Trieste, chances are you're done. Take some wild swings and let me know how it went.
2. The juggernaut forms
If there is a clear juggernaut, DO NOT ACKNOWLEDGE IT TO RUSSIA OR TURKEY! Let them think they're clever. Meanwhile, scream from the rooftops in every other private message about the danger on the horizon. Your best hopes are in England and Germany. Form a strong alliance with Italy, convincing him that he's going to be decimated right after you are if this thing gets rolling. Do everything you can to keep Galicia unoccupied by Russian soldiers and get an army to Syria as soon as possible. Quick, decisive, coordinated action is the only way to stop this. It can be done, but be wary of the Italian stab once you know you can defend yourself from RT.
3. You're doing well
Austria is often out of the game by early 1903 or so. If you're thriving, don't get cocky. I find the best time is when Turkey is about to be put out of his misery, make sure you have a friendly and occupied Russian and sever a few nerves in Italy's back. You should have built a fleet by now. If you've done well enough, Venice is yours, you have a fleet in the Ionian and you're building a second one in Trieste in the winter after the stab. Hopefully France or England is yet to wrap up the West. Get an army to Piedmont as quick as possible. Take another Italian center or two. You don't need to get your fleets into the MAO, but you want to make sure there are no other fleets in the Western Mediterranean either. I'm currently playing Austria. England, Italy, and Turkey are dead. I have a Russian ally... or I did. Germany and France have worked together very well. I am the largest, and am trying to get it down to a 3 way draw that the other 3 aren't interest in. I couldn't beat France, Germany, and Russia. That being said, I have set up a blockade in the Mediterranean. The only way France can touch me is by marching through Germany, which would likely accomplish my 3 way draw. Fleets are of incredible importance in winning or dictating the terms of the draw. You need to build them as quick as you can or the Italian peninsula is nearly impossible to control.