I have the same issues, here's how I tackle them:

For things like the library book, dry cleaning, etc, I have an @Errands context. It's similar to your @If context, except there are two times a week I set aside to perform errands. When the appointment with myself to do errands comes up, I look in the folder and plan accordingly. I carry this folder with me, and everyone knows if they need me to go to the store for something, they just need to drop it in my inbox, the start of my system.

For physical things, and things destined for a specific delivery, like the thing you have to give your friend, I make sure to put them in plain sight, and in my way. For example, if I have a library book to take back, there is a note in my tickler file for the day I need to take it back (or maybe the book goes in, it depends). Then, when I'm getting ready for the day, the book, or note or whatever goes under my keys. Can't forget it if I'm leaving the house. When I get in the car, the book doesn't go somewhere else, like a bag, it goes in my lap, or I carry it in my hand. Luckily, I haven't had a day where I had to deliver something HUGE. :) For a physical item, put it in your way, and you can't forget it. Oh, and when you're getting ready to leave the office for the day, but you aren't quite ready to leave yet, put the stuff you need to take with you in the doorway of your office or cube. You'll have to trip over it or take it with you.