If you're working naked (without a table or servante) where do you plan to hide the second bottle? Your best bet is to have a place nearby to ditch a bottle and retrieve a different one. If you're using a Topit (the only other way I can imagine doing it) then you'll have to deal with carrying one of the two bottles throughout your entire act.
I'm thinking about doing it outside, maybe with a "table" behind me or something, I'm not entirely sure.
What do you have in mind? An instantaneous switch? Or something you can do on an off beat? If it's the second, remember that time is your friend. If you can switch it when the heat is off and they can continue to believe it's the same (first) bottle for a while, then the "magic" can be separated from the "moment."
My original thought was to switch a shorter bottle with a longer bottle in the fashion to make it look like I'm stretching the bottle (with fluid inside this could look even cooler). Another thought I had was simply to switch two bottles of soda, say Sprite and Coke (where it's very noticeable). And either instantaneously or on the offbeat is fine, both is better actually. I just thought of now, it would be cool to show them you change the color of the cap, and then show them you also changed the entire bottle, or start with the cap or something...I dunno, my ideas will only elaborate...
Toppit is about the only thing I could think of for that. Do you wear a jacket?
Sometimes, I would need to get a new one, but it wouldn't be a big deal to wear one.
A standing bottle switch wont be easy but it can definately be done with a topit and some misdirection. Will the bottle/s have fluid in it?
I have no idea what a topit is, but I'll look it up, and I'd prefer to be able to borrow the bottle(s) (At least one). So yes, they could have fluid in them.