For every half-finished piece of cake and happy meal you get to amicably polish off, there will be a half-eaten sucker and melty granola bar you also end up eating because your child handed it to you and there's no-where to throw it away.
Never assume it's the baby that smells like peanut butter just because she put it in her hair at lunch, as it's probably on your shirt/neck/hair as well. And lunch was eaten before you went out.
Only make threats you're willing and able to follow through with. If you said you would put them in time-out if they didn't stop it right then, you sure as hell better be willing to put them in time-out right there in the middle of the grocery store when they don't stop it.
The crying baby always sounds louder to you than everyone else. Unless of course your child really is that loud, in which case your best bet is to always have a package of earplugs in the diaper bag and offer to share when necessary.
The best toys you will ever buy won't actually be toys at all. See: iphone, small rocks, pennies, laundry hamper of dirty underclothes.
Fashion sense is relative. Just perhaps not related to yours.
They will always remember your offhand comment about "sure you can have ice cream on Saturday" but will have no idea where they left their shoes 10 minutes ago.
Preschoolers dislike you falling asleep while putting them to bed and drooling on them and/or their pillow. Just remind them of how many of THEIR bodily fluids have been on you and/or your pillow from those first few years, and explain that karma is a bitch sometimes.
Don't use terms like "karma is a bitch" with your children until they're at least old enough to cuss correctly. Otherwise you'll spend years correcting their poorly executed fucking interjections.
Scold the teenager for saying fuck first, THEN correct their grammatical usage of the term.
Pick your battles, and fight to win. However, actually out-willing a 2 year old is still a hypothetical outcome.
Attach as much importance to your child's lovey as they do. This will decrease the likelihood of forgetting it at the bank. Small children do not understand "banker's hours" and that you CANNOT simply go get their lovey right then at bedtime when they noticed it was gone. But don't worry, you still have those earplugs in the diaper bag.
Never trust a toddler. Their brains are small, and possibly suicidal.
Try to wear shoes with good treads. You could need to sprint after a suicidal toddler or defiant preschooler at any time. Even if you're wearing heels.
Do not wear heels to the playground. You may not ever make it out of the gravel alive. Or at least not without a broken ankle.
Snot is nasty, but not as nasty as some of the other stuff that will end up on your shirt over the years. Therefore, don't stress over the need to use an edge of it to take care of that runny nose in a pinch.
However, it is still preferable to use their shirt for said snot wiping task whenever possible.
Just keep telling yourself that everything will be different in 6 months. Not necessarily better, but definitely different.