How do you avoid this?
Okay so five months in advance you have a wedding booking, all said and done. Say its for September 30.
Then two months in advance you have a large party cake for the same date. No problem.... but customer is still trying to figure out exactly what they want. They tell you they want a cake but then you don't hear from them for another month while they discuss it over.
Then you get another order for the same date from someone who is a repeat customer who wants another cake but the finalities haven't been decided nor a deposit put down.
Then the second person writes back pretty sure they know what they want now.
So ends up you have three cakes for the same day...something that is possible, but something I personally try to avoid just for quality of work purposes.
I suppose I'd have to be more forceful about taking deposits but on the other hand, I don't like to push people into giving a deposit 2-3 months in advance if they aren't ready. On the other hand, if we've discussed the cake, I hate to tell them "Oops now I'm overbooked, sorry."
Is this just "one of those things"?
The first time you talk with your customer use something along these lines.
"If you are not ready to place the order today please take your time to make decisions. I currently have the date available, however if someone puts a deposit down for the day before you I may be full when you are ready to place the order."
I don't think this is pushing it is just letting them know (a disclaimer of sorts) that you are a business and you are going to serve the customers who come to you first and know what they want.
I am sure it could be re-worded (and probably should be) but something along those lines should help.
I would only hold for people who give deposits.
If they need time to figure out the final details fine. I would tell them if they want to secure the date they need to give a non refundable deposit and they need to have the final details X amount of time before the event.
Tell them if they don't want to leave a deposit hopefully you are not booked for the day and can still work for them, but you have to book by deposit only.
Oh okay, yeah I should be doing that.
Sometimes I get an "order" from which the person like disappears for weeks and I assume they changed their mind...only to get an email saying "Ok we know what we want now." And then its almost like a last minute cake because I had forgotten about it.
I usually write down repeat customers orders on my calendar because I know a deposit will be put down soon but if its someone who is extremely indecisive I don't write them down...which is probably bad because sometimes they decide they want a cake. However I have in the past written cakes down and never heard back again so who knows...
I lIked it very much!
I don't like to push people into giving a deposit 2-3 months in advance if they aren't ready. ....... Is this just "one of those things"?
No, it's not "one of those things". To be blunt (and I'm kinda known for doing that 
) you have to decide how you are going to run your business.
You don't like to "push" people? Goodness, I had my 7-7-07 weddings booked over a year in advance and I had deposits from all of them a year in advance. If they want you, then they need to book you. And booking you means a deposit. Money talks .... BS walks.
On the other hand, if we've discussed the cake, I hate to tell them "Oops now I'm overbooked, sorry."
"Discussing" a cake is not the same as "booking" a cake. You can discuss a cake from now until doomsday, but if they don't book it, then sorry, the date is no longer available.
Dont' feel bad because you're overbooked! Why should you feel bad because THEY are not organized? If you look on my FAQ page on my website, I have a question about "how soon should I book?" The answer is "It depends on how organized the OTHER brides are." If the other brides are decision makers and are not afraid to get things finalized, then they are the ones who are going to get the date they want. Those who fiddle-fart around will have to deal with the leftovers.
In the words of Abraham Lincoln:
"Good things come to those who wait, but only the things left by those who hustle."
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