It sounds like your Fondant is too soft and/or your ambient humidity is high. The easiest thing to do is knead in a 1-2 teaspoons of a gum such as CMC (i.e.: Tylose) or Gum Tragacanth per 8 ounces of Fondant. Simply knead the powder directly into the Fondant, wrap it tightly in plastic wrap and let it rest on the counter for about 15-20 minutes. This will firm up the Fondant and allow it be modeled more intricately. Please keep in mind that after you add additional gum to the Fondant, it will dry much more quickly; therefore your 'play period' (the period that you can play around with it before it starts to crust, crack and dry) will be reduced. However, I often find adding the additional gum is necessary for anything that has more than simple dimension. As an alternative, you can also mix your Fondant with Gum Paste using 50% Fondant and 50% Gum Paste. This creates a paste with similar properties to the Fondant with additional gums kneaded into it that I described above.
Hope this helps.
-Adam