I have to admit, I found the first 50 pages really tough going, and it was only due to my stubborn determination to finish a book once I've started it that I kept reading. But after that suddenly everything clicked; granted, there were still slow moments later in the book but I started to really enjoy it. Basically you've got to surrender yourself to the fact that you're not going to understand half of what Sterne's going on about, and that there is essentially no chance of getting a coherent narrative out of the book. But that's the thing that you end up loving most: for instance, when Sterne cuts off a conversation between Walter Shandy and Uncle Toby in mid-sentence and then says something to the effect of "but before I go any further with this conversation, it's important that you understand something about Uncle Toby" - and then spends the next 30 pages talking about Uncle Toby, before returning to the conversation at the exact same point.
Also, not to sound patronising, but as you get older you start to appreciate different literature. I first read Thomas Hardy, for instance, in High School (for class) - and hated him. I returned to him last year, almost 10 years later, and absolutely adored what I was reading. I think if I'd tried reading Tristram Shandy when I was in High School I probably would have been bored stiff, too.
. . . Also, a healthy appreciation of toilet humour helps a lot.