Depends on the kind of lasagne. If it's just regular lasagna, I'll serve a lettuce salad or green beans. If I make spinach lasagna, sometimes I'll just serve the lasagna on it's own without a sidedish.
My sister makes a really awesome spinach and strawberry salad with blue cheese crumbles that goes great with lasagna type things. Of course if strawberries aren't in season then that dosen't work. I like a good greek salad too. Really any salad that has a vinaigrette with good zing. Garlic bread is a must as well.
We always have green beans (which I cook with butter, bacon, & garlic), corn on the cob or whole kernel corn, and either spinach salad or just a green leafy salad. And garlic bread, usually with cheese!