So for example, instead of real rose heads placed lovingly on and around your cake, why not choose silk ones?
DO mix it up, cut the stems down to size, cut pre made bunches up.
DONT put short stems in long vases, I find the stem looks the most fake part of the plant so if you can avoid them and only use squatter smaller vases than you will get a more realistic look. Let the hubby buy the real long stem roses for you as if you have them already that's just another excuse for him not to!
I have these from GT decorations which I have separated and placed in clear short vases to brighten up my shelving.