It depends on the rose. Some roses will open quickly when the stems are recut and put into warm water, or put into a warm room. At a minimum, the sepals must be down and the first row of petals starting to open. Some roses won't open no matter what. some open so fast they will go past exhibition form in a few hours. Depending on the stage of openness, if it's close to opening, then you can groom the petals to open using cotton swabs or cotton balls (see Bob Martin's book, Showing Good Roses for more details).

If they are starting to open You can speed it up a little by drinking hot cofee and blowing into them, Looks stupid, But I swear it works!
Lee

Or you could try holding a gun to their head and threatening their life. :o) This would be almost as effective as most of the other pet methods people support. Unless they are already inclined to start opening, there isn't much you can do.