Using low fat alternatives could be a big problem. I don't think the Mornay would work very well from skim milk and non-fat cheese substitute. Probably most of my ingredients are at room temperature (not the butter which obviously doesn't matter) but I'd probably add cream from a carton that's been out of the fridge and on the counter for 20 minutes. Maybe I'm just lucky. It's often the case that being lucky trumps being skillful.
One thing for sure, I'm no expert, although not a novice either. I probably began making sauces seriously about several years ago, back when I really started to get into Julia Child's books.
Another thing to keep in mind is to add ingredients a little at a time. I naturally do that because I'm not measuring things. I'll add some ingredient, incorporate it, then taste it to see if I need more. If it's cream for example I wouldn't add so much that it would shock the temperature of the sauce. I would also probably have the sauce fairly hot since I stir it almost constantly. I usually make the sauce at the end so everything else is probably cooked and the sauce becomes my sole focus. It's the last thing I need to get done so that I can serve.