Your veterinarian or veterinary technician will show you the proper way to administer subQ fluids. It looks easy in the office, in the hands of a professional; but remembering all of the techniques for the procedure while managing the patient at home can be intimidating.
Here is an excellent step-by-step photo tutorial for administering subcutaneous fluids in a cat (also works for dogs):
Giving Subcutaneous Fluids to a Cat
From Washington State College of Veterinary Medicine
Now to the "tip" part. I recently attended a feline Continuing Education (CE) class presented by Hazel Carney, DVM, MS, DABVP who strongly urged veterinarians to warm the bag of fluids rather than administering the fluids at room temperature. Most of us just have the bag hanging, ready for convenient administration.
Since my beloved Herman Roosevelt was diagnosed with hyperthyroidism and early kidney failure, he needs an occasional boost of SubQ fluids. I have found this tip to be invaluable. I admit to usually just giving fluids at room temperature. The majority of cats and dogs never seemed to mind or notice. Herman is so easy going; a cat who opens up his mouth and sticks out his tongue to take his pills and hands out his paw for nail clippings. Needles and SubQ fluids are another matter. All of a sudden, his going-on-17 year old body has the strength of a three year old and fluids were going everywhere but in the SubQ space.
I wondered a bit at how best to warm the fluids, finally settling on my biggest cookware pot. I fill it with hot tap water, and usually top it off a bit with water from the kettle (never directly on the bag, but in the full kettle of water).
Dr. Carney recommended using a flexible peel-off thermometer, like the kinds used for fish tanks or to check the temperature of a child's forehead. They are inexpensive, and can be found at pet and drug stores.
I did the "wrist test" -- running a little of the fluid over my wrist to make sure it "matches" my temp, not too hot and not too cold. It would be best to use a thermometer to get an idea of the ideal temperature; 98-100F for your pet. Fluids that are too cold will be uncomfortable for your pet, fluids too hot may burn/damage the SubQ space, so caution is advised. Always use a thermometer if you are unsure.
I also give Herman his fluids when he is hungry. I open up a fresh can of his favorite food, one with lots of gravy, and let him partake before starting in with the fluids. This puts a positive spin on everything, and he doesn't even notice the fluid administration.
Using warmed fluids (and some freshly-opened canned cat food) make giving Herman his fluids a treat and not a chore. For both of us.
Addendum - October 2008:
Sadly, the time came to say goodbye to Herman. Fluids helped keep him comfortable, but eventually, the hyperthyroidism and kidney failure took their toll. I bid my goodbye in this article: "Twilight Time: The Long Goodbye." I miss him.