I use whatever is in the fridge or pantry. Fortunately, there's usually jamon serrano in the fridge. As for condiments, I love Japanese mayonnaise (it's so much better than regular mayo - there's a touch of tangy, sweetness that's so unique) and some chili sauce for a little kick. Plus, some fresh herbs, like basil, further liven up the sandwich.
Paired with my sandwich is usually a salad or some atsara (pickled vegetables).
Check out my Flickr album if you can't see the slideshow here.