When can you begin potty training?  Anytime.  With Jesse, we started her the first day we had her (she was about 3 months old).  We were seeing results within 3 days!  There's no need to worry about beginning too early - you aren't going to make your bird neurotic by starting too early (you just won't have a lot of success if he/she isn't ready).  It takes a while for a young bird, however.  Very young baby parrots, like human babies, need a little time to recognize the feeling and have enough control to "hold it".  But once they can connect the feeling to the action, you can train them.  Know that unweaned babies' bowel movements are more frequent (and irregular) than weaned birds' are.  The bowel movements will become less frequent after weaning - and more predictable/regular, too.  

All you have to do is wait for the her to "go"...and then say "OH, LOOK!  EVERYBODY LOOK!  (insert bird's name here) made a POOOOOOO!".  (For "poo", substitute whatever you are going to use for your command).  After 3-4 "poos", she'll start to realize what she is being praised for.  Birds are v. quick to understand the "topic" in this.  You'll know you're ready for the next step when the bird hears your praise and looks at the dropping (as if to say "Oh... yes... there it is...").  The tough part is not to laugh when they look at you as if to say "Aren't YOU easily amused...?"

The next step is to watch for when she's just about to go... then say "(bird's name), make a poo" - and then praise her when she does.

Just be consistent.

The objective is twofold:

1. Get her to evacuate on command (easy). Once she's doing that, don't praise her for every poo - just the ones she did on command.  
2. Get her to "tell" you when she has to go.  THAT part - wonderfully enough - actually happens on it's own.  The bird will give you body language that lets you know it's coming.  This is where YOU get trained (grin).  She'll eventually give you enough "warning" so you have a few seconds to get her over some newspaper or whatever.

And... just wait until the day when she tells you she has to go by saying "poo"... yes, that will happen, too, although probably not consistently enough to rely on it 100%.

Of course.. when an accident happens, never scold.  Scolding only upsets them and doesn't really accomplish anything.

Oh.. and about the praising after poo-on-command... don't ever stop doing that.  It reinforces the behaviour.

Chose The Command Carefully
What I'm about to say may seem obvious, but I'll say it anyway.  The potty command needs to be a phrase or word that doesn't come up in everyday conversation - and it needs to be something you'll be comfortable saying regardless of who's nearby.

Once your bird is potty trained he/she will perform the action (if possible) each and every time the command is given.

Give The Command Consistently (why Mom gets better obedience than Dad..)
A command is a command.  It should never be stated as a question or request.  When you give the command, do it with a firm (not angry, but FIRM) voice.  And say the command in exactly the same tone using the same phrasing.

I have noticed that I seem to get better compliance than my husband does.  I am convinced that this is because I always say the command in the same way: "Jesse, make a potty."  My husband varies his commands, and sometimes states them as questions "Jesse, let's see our girl make a potty now."  Eventually he gets results, but they respond faster to me.

This is especially noticeable with our conures, who seem to play the "I don't have to if I don't want to" game with Stephen.  Quite frequently, Stephen will issue the command (using his words) and after no results happen, I follow up with "Bubba, Make a POO!" and then a stern "DO IT!"  All the kids seem to know when Mom (me) says "DO IT!" it's time to surrender and do what was asked.

Be consistent, be firm, use the exact same words each time.  You're looking for a "Pavlov's Dog" response.

Alternatives: Special Poo Places
It is possible to train a bird to go only in approved places.  I've read, however, that trying if you try to train a bird for both "do it on command no matter where" and "do it only here", the results aren't as good (the bird gets confused).  I believe there's some sense to this: what you want is instantaneous cooperation - not a bird who's confused.

Look at it from the bird's viewpoint.  Polly is trained to go only in her cage or on her T-Stand.  But now she feels the call of nature, but there's no stand or cage nearby.  You give the "potty word".  What is she to do?  Obey now?  Wait?  What's the right thing?  Hm... maybe flying to the cage or T-Stand is the thing to do?  Yes!  That's it!.  And so she begins her flight, but just doesn't make it all the way and ends up having an accident because she waited too long.

You don't want a bird "in need" to be sitting there trying to reason out whether it's OK to go or not after the command is given. 

I've had pretty good luck with getting my birds to recognize that some places are better for "potty" than others.  But that happened more because I tended to set them in the area before giving the command, not because I focused them on the command. 

I've heard stories about parrots who were trained to fly to a human toilet and go there.  I have no idea if this has been done, but I have two reactions.  One is that it sounds too much like "urban legend" to me.  The other is that I really don't want my birds to be flying over to a place where they could fall into water and drown.  It sounds dangerous to me.

Focus on the command, not the place.  You'll get more consistent results.

Alternatives: Flight Suits
There are "flight suits" available for purchase.  Kind of a "bird diaper".  It sounds like a great idea - and does have attraction that such a suit might make it possible to take the bird to places where a "poo" would be a big problem.  

I haven't tried one on any of my birds because they prevent the bird from fluffing feathers - and parrots must do that to control their body temp.  Overheating is a very real possibility with a flight suit (plus the suits are just plain ugly).

Potty Training in a Multi-Parrot Home
For those of you who, like us, have mulit-bird households.  Consider having different poo commands for each bird.  Our parrots are VERY obliging... and when they get the command, they try to "give up whatever they've got" immediately.  I read a story once about a woman who had all her Cockatoos (large parrots... big poo's) trained on the command "go potty".  She had 3 Cockatoos sitting on her couch and took out the fourth to join in play time.  When she took out that last bird, she held him over a trash can and said "go potty".  He did.  And so did the other 3 (on the couch...).

So all of my birds have a different command.  It's worked out beautifully.  Bubba gets "Make a Poo", Forte's is "Make a Grunt-Grunt", Pakshi's is "Make a kakaboodee" (Ok.. go ahead and laugh, but nobody ever says that one by mistake), and Jesse's is "Make a Potty".