With dd, we always had the tree (it's an artificial one, about 5ft tall) up on a small table or something, 'barricaded' into a corner by the furniture, so it was basically out of reach. Since she wasn't much of a climber, we didn't have any problems with that.
Ds, however, is something of an adventurer, so last year we had to come up with another option. Ds was 14 months old and, having moved house, we didn't have any corners to put the tree in. Instead we had it in plain view and positioned in front of a radiator. My main concern was that ds would pull the tree over on himself, so I tied two pieces of string to the trunk part of the tree and tied the end of each string to opposite ends of the radiator. It probably sounds a bit weird, but it worked as the tree was anchored securely. After that, of course, it was a matter of only putting unbreakable, child-safe ornaments on the tree, and then reminding him not to remove the ornaments, and helping him to put them straight back on if they 'accidentally' fell off. We didn't have any probs at all.
This year, of course, at just over two years, ds is even more of an adventurer, so we shall have to see how it goes, but hopefully the same strategy will work.