With bowling, its how you rotate the wiimote that affects the spin. E.g. hold out the wiimote as if you're pointing to the screen/shooting a gun straight. Now rotate your arm so its like your holding your gun sideways ("gangsta" style!). Now if you hold it like this your ball with spin. Hold it straight without rotating your arm and it goes straight.
Tennis is too good, me and my bro are wicked at it now, always putting top spin on the ball to make it go fast. You have to get used to the controls!
Never got used to baseball but dont like baseball anyways. Boxing is good but haven't played it enough as I've only got one nunchaku and I always play with my bro. Golf is good, the better you get the less you need to practice swings before you actually take one.