How can I make toilet training a positive experience for my 18-month-old?

Signs of readiness. are being able to walk from room to room, pull diaper, pull-ups & pants up & down, be dry for 2 hours, say "potty" or other word to indicate he senses the urge to go. Till then, let him sit on a potty, clothed, while you model using the toilet. Pushing it creates behavior problems. Buy a Little Looster stool. Read http://m.kidshealth.org/parent/emotions/behavior/toilet_teaching.html.
+ reinforcement. Kids repeat activity if rewarded in a positive way. If strictly using pavlovian training you can make the behavior repeatable by means of a positive association, such as smiles, claps, new undies, or playing a game. Any negativity or disapproval associated with the opposite behavior can affect the result. Keep it low key and positive!
Relax. In normal kids, everyone eventually is potty trained. This is a normal milestone of development.Thus it will always happen. Do not pressure your child. The average little girl trains at 26 mos/age and the little boy trains at 32 mos.Train by example until the child is ready.
Show them. Toddlers are naturally curious about the use of the bathroom by other people. Let them watch you or other family members use the toilet. It is important not to put too many demands on a child or shame the child during toilet training.