He didn't earn one sticker in the first 3 days - that's how bad our backslide was! We then tried compliance training - in other words, rewarding him with M&Ms for using the toilet when we told him too, and that worked so-so. He finally earned his first sticker this way, but then un-earned it by having an accident right before we put him to bed! I tried to remove the sticker from the sheet to make a point, but it was stuck on really well, so I ended up spending five minutes ragefully picking it off piece by piece while Jackson cried. Not exactly high-quality parenting.
But the most effective intervention came when we started enforcing regular potty breaks after meals using the inspired little poem, "After we sit at the table and eat, it's time to sit on the potty seat." Jackson was a little skeptical about this at first, but after a couple of days, started enthusiastically using the toilet four times a day (three meals plus a snack) while reciting this poem. Combined with scheduled trips before nap and bedtime, this seemed to do the trick and he quickly earned all 10 stickers needed to get his horses, and we haven't had an accident in weeks! We were so excited that when he earned his stickers he got THREE horses (Mommy, Daddy, and Baby) instead of two.