make sure your fluids are warm enough, if the fluid is cold then it causes pain when it goes in. if you are using a different bag each time, then you can store the sealed bag at room temperature and give it at room temperature.
if you are using one bag and drawing fluid from it each time, then i hope you are storing the iv bag in the refrigerator. once the iv bag is punctured it has to be stored cold or else you risk bacterial growth in the fluid.
clean the iv bag port with alcohol, draw up the amount you need and then warm the syringe. you can either roll it gently in your hands (takes a while), hold the syringe over hot water so the steam warms it up, or else if you have the kind of syringe that comes in an outer plastic casing you can place the syringe back in the casing and place the whole thing in warm water to heat up. you do *not* want to put the syringe directly in water because of contamination of the needle area.
also, you can give the fluids anywhere alongside the spine. not close to the spine, but on either side. sometimes it's ezier to hold or wrap the bun so his rear end sticks out (but back legs tucked & wrapped) and you can make your tent in the lower part of his body. and again, it's better to have 2 people if you can.
make sure you give treats & lovies after you're done! 8-)