Never Use However at the Beginning of a Sentence.
Here's another tip from Bryan A. Garner's The Winning Brief: 100 Tips for Persuasive Briefing in Trial and Appellate Courts.
Garner instructs brief writers to never use 'however' at the beginning of a sentence in order to indicate a contrast, and recommends using 'but' instead. William Zinsser's On Writing Well gives the same advice: "I can't overstate how much easier it is for readers to process a sentence if you start with 'but' when shifting direction . . . Don't start a sentence with 'however' — it hangs there like a wet dishrag." 'However' can be used at the beginning of sentence in the sense of 'to whatever extent'.
If you want to avoid using 'but' at the beginning of a sentence, shift 'however' to the middle of a sentence, or rephrase it using 'although'. For example, sentences like these:
In his "Statement of Disputed Material Facts," Plaintiff lists numerous "facts" he presumably believes show a retaliatory intent. However, Plaintiff never attempts to explain how these facts show retaliatory intent."
. . . should be rewritten as:
Although Kautz's "Statement of Disputed Material Facts" lists many "facts," he never tries to explain how they show any retaliatory intent.
There's no reason to shy away from using but to begin a sentence. The most eminent of legal authorities do this:
"But Marshall ruled to the contrary, holding that the court did have the authority to require Jefferson to produce the documents in question." William H. Rehnquist, Grand Inquests 117 (1992).
"But what if the parties fail to raise any jurisdictional question?" Charles Alan Wright, Law of Federal Courts 28 (5th ed. 1994).
A comma should never be used after a sentencing starting 'But'.