Brush it off, don't beat yourself up, and just get right back on track. Don't be like "oh well I already blew my week" or something.. get right back on track today. 2 days in a lifetime of change, or a 2 lb fluctuation in the long run will be nothing provided you're still sticking to your good eating habits a large majority of the time
Also celebrate your good choices. If you were going to drink soda, at least you went for the diet kind over the sugar loaded calorie heavy stuff. That's a win, even if a smaller one.