Well, this is one information for which i searched all over the Net and could not find it, which is why I am making this page.
It is very simple to go from Delhi to Ajmer, visit the Ajmer Sharif Dargah and come back to Delhi in a day, though it could be a little tiring.
It is not possible to go by car, as it involves over 7 hrs drive one way. The best and probably the only way to do it is to take the Ajmer Shatabdi from New Delhi Railway Station. It leaves at 6.05 am and reaches Ajmer at 12.30pm. Get out of the station, on your right you will see an overbridge crossing the road in front. It is a very dirty bridge, so if you can cross the road do it, and reach the other end of the over bridge. In front of you is a T Junction, go to the right and almost immediately you will reach a road junction. Ask anyone, who is not interested in taking you to the Dargah, like a shopkeeper, the road to the Dargah, it is the main market road. It takes 10 mins on an average speed to reach the Dargah and you walk through the beautiful colorful market. If you cannot walk, then an autorikshaw should take Rs 40/- to drop you at the nearest point of Dargah while a cycle rikshaw takes Rs 30/-. But if you can walk, do it.
There is plenty of time as the return train goes back at 3.50pm. Spend as much time as you wish in the Dargah, then walk back, shopping on the way, if you wish. There are three resturants in the road in front of the station which give good clean and cheap food. Have lunch there. You will still most definitely reach the station well before the departure time of the Shatabdi back to Delhi. You are back in Delhi at 10pm. You get well fed on the way, the price of food is included in the fare both ways.
Have fun !