What do you do when you don't know what to do?

So it takes hundreds of years to find the best opening lines?

Nimzowitsch's advices and ideas can still lead to decent wins on this very day

Are ALL the famous chess players dead?

Matthew 11 asked Chess never changes, how can openings be outdated?

To clarify a bit here, specific opening analysis such as most of the specific opening analysis/recommendations in a book as old as My System (©1930) should either be ignored or carefully compared to something more up-to-date for instance analysis of the same line(s) in NCO ©1999 or MCO-XV ©2008; and/or compared to current examples of master play from a recent database.

BTW a few openings are "outdated" such as the Damiano Gambit and others are unfashionable (such as the Danish Gambit, Poniziani Opening or 4 Kts Game) because they fail to give White little or no advantage vs accurate play, except for the cases in which they actually end up giving White an inferior game. Other examples include the Latvian Counter-Gambit which is practically busted for Black vs accurate play by White.

The rules of chess don't change (aside from a few more exceptions to the 50-move rule and drug testing) but the theory of openings changes constantly.

That makes sence, there are 6,000,000,000 positions after 3 moves.

I think we've had enough echecs06 today

My Advice :

Do play Your Position a dozen of times till the mate. Then Position opens It´s Secrets to You. Do it 3 days in a row & sleep good nights between. I am sure You´ll have an Executable Plan after this Process.

If it´s a Tournament or Team Match Game, do check a BIG PICTURE, too.

Studying Opponent´s previous games is always  recommendable.

Good advice, you mention learn the pawn structures and the "evident" plans otherwise.  If you can't do a plan, improve a piece.  Those three pieces of advice will take someone far.

I'd like to add when none of your pieces are apparently "bad" (and you can't undertake anything concrete) then it's enough to slightly improve one piece here and there.

Also be extra careful about what you opponent can do due to the change you're going to make in your position.  This is usually when a careless mistake is made.  So instead of making "is this good for me" your primary focus (you already admit you can't find anything really good) make "what does this give him" your primary focus.

The flip side of this is, if it's not safe to do anything, and you're facing a sub-master, try being a bit stubborn and mentally prepare yourself to repeat the position a few times (make sure they can't undertake anything useful either).  A weaker player usually cracks and hurts his own position out of the need to "do something."

My two cents.

bumper sticker: What would Tal sacrifice?

WWTS? I want one! Make sure it has eyes giving a piercing stare.

try a waiting move and see what comes along.Now is not the time for "hmmm lets try this".

In my opinion the most important point when you don't know what to do is make sure you DO NOT dream up some wild/risky/speculative plan.....as others said develop your worst piece, trade an opponents best piece, stop their plans - play simple chess and a move with no downside....just make sure you don't implode & self destruct. 

If you don't know what else to do, don't move a pawn.

As Korchnoi once said, the sign of a strained strategist is he starts moving his pawns, making his position worse.

Even so, at least in the spirit of erring on the side of activity, maybe it is not so terrible to go for a pawn break whenever a position doesn't seem to be offering you anything.

(although we might say a million exceptions apply... so I don't know how good this "advice" really is haha)

When you don't know what to do, maneuver. Try to give your worst-placed piece a useful role, even if it's just taking over a defensive task to free up a more active piece.

How do you decide on a move when you don't know what to do?

Find your least active piece, and make it active.

Keep up the pressure.