I have one for you Jovimar!

I got this exercise out of a health magazine and try to do it everyday.Sit in a chair feet shoulder length apart,feet straight,and back straight, put your hands on your thighs.Squeeze your buttock for 10 quick reps.,on the tenth rep,hold it for 10 seconds,then release. Do this for at least 2 minutes.I sometime do more reps. per set and increase the time.At the risk of sounding immodest I've been told I have a nice derriere

so this one might work for you.I incorporate this with other rear exercises but this one is good for a quick toning before going to work, at work or wherever you might be sitting.I've attached a picture which seems to be another good exercise.Good luck!