Having recently started knitting a new sweater I realized that there are a lot of little things that I do, that are worth collecting in one place:
Do not tie a slip not to start, simply twist the yarn around the needle as a basis for casting on the first stitch. This twist looks like a stitch, but isn't, you should decrease it at the end of the row, with the last stitch to complete the join.
Always used the German Twisted long tail cast on variant, which makes things a bit more elastic and just looks great, particularly when knitting ribbing, which I often do at the beginning of a sweater.
Wrap the yarn around the needle once per number of stitches that you need to cast on to estimate the length of the long tail that you'll need to cast on. I find this overestimates a bit, but I've rarely regretted having too-long of a tail rather than having too short. While you can start again from the beginning in the case that you run out of tail, you can also splice in a second yarn.
If you do run out of yarn while casting on for the sweater, and you've been using the long tail for the finger yarn (loops around the needle,) you can sometimes get a few extra stitches out of switching to having that needle
Place markers periodically to make it easier to count, roughly every 20 stitches or so, and I try and make sure that one of the markers gets placed half way through the round. For example, to cast on 228 stitches, I placed 12 markers every 19 stitches, and the 6th marker was the "half way" point.
I did one sweater where I put markers every 32 stitches and one sweater where I put markers every 16, and found that I spent far more time casting on the one with fewer markers because I had to double check my counts more. They really help.
Cast on to a needle that's a bit bigger than the size you intend to use. I've been quite happy using a US 2.5 to cast on for a US 0 sweater. I've been using interchangeable needles, and being able to replace the larger needle for the smaller needle before beginning to knit has made things much easier to knit for the first row. It's also an option to hold two needles together for the cast on.
I also have to think about not pulling on the "thumb yarn" at all, as this will also make things tigheer.
While it's good to be careful to avoid twisting the first row, if you do accidentally twist, undo the twist between the last and first stitch, which will hardly be noticeable.