The Northern hemisphere vacation months of July and August are busiest for foreign travelers, so prices tend to rise for guides, accommodation and tour agencies. It also means that you may need to reserve tours and hotel rooms in advance.
The region is also quite busy in January and February when hordes of Chileans come north to spend their summer vacations at the beach. Again, you may want to book in advance.
Between December and February, the terrain becomes much less conducive to travel, as heavy rains wreak havoc on roads and it gets much colder in the Altiplano.