A few simple rules
In all classes blooms should be fresh with no blemishes including insect bites. Fresh also includes the stamens and pistils. Foliage should be fresh, clean and free of damage. You should remove any damage but be careful to make sure you do it in a way to minimize the effect on the entry. For example take off damaged blooms close to the stem rather than leaving a leafless stem on the branch.
If you need to cut off part of a damaged leaf make sure you cut it so that it appears to be natural by following the shape of the leaf.
Overall appearance is important. Try to make your exhibit as pleasing as possible. Sometimes this can be difficult as plants can have a mind of their own but you can use filler [cedar or juniper works well] to hold the stems in place. Technically, any filler should be below the top of the vase or bowl [in a regular show anything above the vase top is judged and can disqualify an entry if it is not included in the description of the class]. However, in our Parlour Show, to reduce the amount of fuss we will let some filler show above the vase but try to keep it low relative to the exhibit.
Remember to always read the specifications of the class. If it says not over 24 inches make sure you measure when in doubt. If the class calls for three stems make sure you have three stems.