My problem with calling an IF is I forget if it stays on for several batters in a row. You have several walks or singles nad it goes to the back of your brain if you don't remind yourself before each batter. I have no problem knowing the situation just remembering after a short time. It's called sometimers disease.
For Balks I have learned to call them over the years. I was a centerfielder when I played so I had no frame of reference when I started umpiring. Thourgh a ton of rule study and clinics I have become the local trainer of umpires and am quite good at the balk rule. It's a litle tougher when you work multible levels because the rules change from book to book but you just have to keep in your mind what level you are working.
The hard ones to learn and properly judge for many are interference/obstruction. They aren't hard rules but many either never learn them correctly or prefer not to call it.
The biggest piece of advice I can give you is to remember that sometimes you have to make calls that some aren't going to like. Many youth umpires don't like to rock the boat. You must get over it and call the book correctly.