Fun Fiesta Cakes ...
I always insert a toothpick or a piece of dry spaghetti to the body/head to give it support. I also wait until the gumpaste has hardened somewhat before putting the head on top of the body – if you put it on too soon, the weight of the top will make the body lean.
Weather – especially humidity – will also affect the outcome. If you are using strictly gumpaste, you will need to work quickly because it will dry on you – so when making figures, I usually use 50% gumpaste and 50% fondant – that buys me time to model the figure without worrying about dryness.
Hope this helps!