How can GM try out macros without them showing in the Chat Archive?

I don't want players to see what creatures and powers I'm preparing. Usually I type macros for most of the monsters attacks and then delete the entire chat archive just so they can't scroll back and view them. I know I could make the macros be gm whispers but then I have to remember to unwhisper them before use.

If this is an issue and I think the majority of players out there wouldn't even think of it let alone bother to scroll up and look, you already know of the only way to deal with it that roll20 has at the moment. Clear the entire chat log. I like to do this after every session anyway just to help keep the memory usage down, they have had some issues with a full chat log causing problems on some machines.

You could clone/extend the campaign then test your macros and after tweaking then you just cut and paste them back into the main campaign. If you were a mentor I would have recommended the transmogifier instead of cut and paste.

If you just want to test a macro you can use either /w GM or /gmroll to make the rolls/text only visible to the GM. If you haven't started your campaign yet, you can do all the testing you want and just delete the chat log afterwards. MK's suggestion of a 'staging' copy of your live campaign is also an excellent idea for testing things that would take a lot of time/effort to undo in your live campaign. (and honestly it's always a good idea to make a copy of your campaign as a backup anyway)