In reply to JohnV:
Most book their whole trip through an agency, guided, and the mules are included in that, but if you're independent you just need an agency as a contact to book the mules and provide tent space/toilet in BC.
I think last time I used Grajales and there was an office right in the middle of town, mostly selling tours but you could just go in and book mules only. You took your bags, on the public bus in our case, to their shed at Puenta del Inca, dropped them off, got them tagged etc and the guy gave us a ride up to the trailhead. Our bags were there at the same place when we got back. Grajales base is in Los Penitentes nearby but they have a shed at Puenta del Inca.
The mules travel (much) faster than you, so on the way back you leave BC at, say, 8am and get into Puenta del Inca at 4pm and your bags have been there a few hours. On the way in, if you stop a night or two at Confluencia you need your sleeping bag, water bottle, snacks, camera etc as your bags will go all the way in to BC in that first day, where you won't get to BC (Plaza de Mulas) until the second day.
I actually carried our tent to Confluencia then intercepted the mules early the next morning and added it to our load to BC, but normally people use agency tents at Confluencia (they were short of such tents when I was there last). We also bought dinner and breakfast at Confluencia, saving carrying food/stove on the walk-in. On that occasion the food was good, but I've seen mixed offerings at other agencies. It might sound a bit complicated but it works out pretty straightforward and is the best mix of being almost totally independent, unguided, cooking for yourself etc, but not crippled by huge loads and being able to enjoy the place. I did carry all my gear out myself once, no mules, 30kg pack for 40km in plastic boots, and that was awful.
The walk in to Confluencia from the road is really easy, about 2.5hrs. But the next day up to Mulas BC is a long one and gains 1000m or so. Start early to make the creek crossings easier, and dress for cold wind and dust. Despite Aconcagua's boring reputation, it's a beautiful valley to walk up, but a long grind.
http://www.grajales.net/aconcagua/services/mules.html