"So you wish to follow the path of power?"
I'm one of these level 20 characters that you guys have been talking about. Adari is my 20th level necromancer, and if I'm not mistaken the first 20th level wizard in PFS. I'm happy to give some high level advice since not many games get to that point.
Your saves are not high enough- This is how you should view your saves at all times. If there's a way you can increase your saves you should do it. Damage isn't what really threatens to kill you quickly like it does at low levels. Save or die is the bane of high level PCs.
Pull everything you can out of 6 seconds- High levels is where the action economy really comes into play. This applies more to casters than physical fighters, due to the need to full attack. Quickened spells are the bread and butter of high level casters as you now have more spells than you'll likely cast in one day.
It's gonna have resistances- If you rely heavily on a single form a elemental damage you need to have a way to deal, other types of damage. High level creatures will often be resistant or straight out immune to at least one type of energy. This is especially true of high CR outsiders.
It's gonna have DR- As above high CR creatures will often have high damage reduction. I know there's a lot of high damage dealers using a two handed weapon out there, but I've seen several instances where a creature survived a full attack with about 20 hp thanks to it's DR. Then it proceeded to unload on the melee character, and kill her (Sorry Naamah but others need to learn from you, especially those without heroic defiance.). At your level you should be able to afford a magic weapon with a high enough plus to overcome most DR, but you should also spend the extra cash to have an extra weapon of each of the special metals silve, cold iron, and Adamantine.
Carry a backup weapon or 4- At high levels your equipment often makes you who you are. As you gain levels things will go after your precious equipment. Don't be that guy who only carries his signature weapon. Because when you lose it you'll be out of luck.
Kill the guy with the pointy hat or holy symbol- High level spellcasters are bad news. Every turn they get is likely one turn closer to them killing you. If you see a high level caster kill him before he can throw out his big spells.
Have a means of travel- When you get higher levels you'll be expected to find your way to rather out of the way places like forgotten tombs, levels of hell, the moon, ect. Make sure you have a way there, and most importantly BACK.
You can fly?!- At this level you had better have a way to fly in some way shape or form regardless of class. There's nothing worse than getting thrown into a pit of lava and not being able to get out.
Be able to take a hit or three- It doesn't matter if your a wizard or a fighter you need hit points. As much as you might stand in the back at some point you are gonna get hit by something big. Make sure a single hit doesn't knock you out of the fight.
"There's your advice from a venture captain who actually knows what he's doing. Any questions?"
Congratz Adam. Because of you a Necromancer now travels the planes collecting creatures he finds interesting enough to want to keep and Cheliax now has 100+ prestige points added to it's collection. Just remeber that when it comes down to it your the one responsible for the start...
But seriously good job Adam you deserve every star you have under your belt.