Getting the Medication Where It Needs to Go
Liquid Oral Medication is generally administered in a 1cc needleless syringe, in the correct measured amounts. Shaking the medication will help ensure the active ingredient is in suspension. After giving your pet his medication, disassemble the syringe, rinse and allow to dry/air between doses.
The best advice I have found for people having difficulty administering oral medications to their guinea pigs is Pinta's.
You can get any med into any pig. You just have to be tough. My method is to prop the pig in my lap with it's back pressed into my ribcage, holding it with my left hand around the stomach and my forefinger and thumb gripping the head/jaws just below the eyes. The grip on the head should be vise-like. There should be no head movement. With the right hand I stick the syringe into the side of the mouth directly behind the front teeth, and work it down to the back teeth until the pig starts grinding on the end of the syringe. Then I slowly depress the syringe contents into the mouth, stopping if the pig stops chewing. As long as there is chewing motion - the pig is swallowing. A little wiggle of the syringe will often get the chewing motion to start up again if it's stopped.
I have never had to wrap a pig in a towel for giving meds.
Make sure you hold the syringe securely. Some pigs enjoy grasping it and giving it a toss.
Pills Try to get the pill in the back of the mouth (from the side) to the molars. A hemostat (a scissor-handled clamp used for compressing bleeding blood vessels, available at a medical supply store) is perfect for dosing pigs with pills, as they can be inserted down to the molars where it is much harder to eject.
Subcutaneous Fluids A butterfly catheter (photo) is the easiest method for administering subcutaneous fluids (use new needles every time). Your vet can show you how. A description (with illustrations) of one person's experiences learning to give fluids to her ferret may be helpful. Read this information on administering Subcutaneous Fluids. Be sure to view all the instructional pages by using the right pointing arrows at the bottom of the page.
Eye Drops Reshevin's vet suggests placing the pig in a table/counter facing away from you. Have the bottle dropper above and behind the head. Try to hold the eye open and apply the drop from the top where they can't see the dropper well.
Note: Antibiotics and other medications should NOT put in drinking water. This is a very ineffective and inaccurate method of treating your pet. If your vet gives you a medication to administer in this fashion, find another vet. 