I've had to do 2 different things with 2 different rats. The first one, Nimue (neem-way from the TV-movie Merlin), who is too smart for her own good refused to take it. Initially I was just holding her gently in one hand in a way that she had a hard time getting out of and gently sticking the syringe in the side of her mount (no big teeth in the way). The worst she usually could do was turn her head away. It took a looooong time to figure out how to hold her. She actually had a really bad reaction to the anti-biotic but thats neither here nor there.
The second one I started out with that way but then I had a clever idea. what I did was I filled the syringe with a little apple juice and fed it to her (until she lost interest at least initially) then put her back in the cage. I did that again a while later. Then, when it came time for the first dose of post AJ meds, first I got two small cups. One with juice and one with water. First she got a SMALL ammount of juice. The I rinsed the syringe our with the water (I also had an empty cup come to think of it for the dirty water). Then I gave her her meds and then more juice. After a day or two she would instinctively latch onto the syringe as soon as it came around no matter what was in it because she was hoping it was juice. Once that started I would just give her the meds first, then the juice. In fairness the juice was paritally because it was good for her and she wasn't eating well at the time (very sick little girl) and I wanted to make sure she got SOMETHING in her. Also please note that the juice was all natural, no added sugars or anything like that. Basically apples and water. She loved it and any time she sees a syringe she goes for it (which is good because now she gotta take different meds for the mites that have popped up).