I always start by leaving them naked from the waist down (or maybe at most a relatively short dress on the girls) in the beginning. This way they see exactly what's going on and it elliminates the extra step of pulling down pants/underwear when they need to go. They'll often remember at the very last second (and even that is a big victory for many kids) so just those couple extra seconds can make all the difference.
Reminding them fairly frequently and making a routine of it also helps. Everyone is different but taking them just before mealtime, or just after, before/after a nap, just upon getting up in the morning, just before bathtime, just before bed etc... You get the idea. Often they don't realize they have to go and this can be a good way of getting some success to help motivate them.
Other than that just be patient, prepared for messes (in clothing and/or on the floor) and let the child take the lead. If they're interested and enthusiastic then that's great, but if they're fearful, resentful or upset then it's ok to take a step back, even putting them back in diapers if necessary.