How do I prevent widow/orphan lines?

You can now use the nowidow package to make this task easier:

\usepackage[all]{nowidow}

As Brent points out, you cannot always do this. The best you can do is to tell TeX that it's infinitely bad for these to appear:

\widowpenalty10000
\clubpenalty10000

One thing to keep in mind is that when presented with multiple infinitely bad options, TeX just picks one of them so you can still get widows or orphans.

The Memoir manual, in section 3.5 "Sloppybottom" discusses this in some detail, which I won't reproduce here.

Be prepared even to re-word in the most intractable cases.

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Update:

I think the specific commands like \enlargethispage and \sloppybottom are exclusively for the memoir package, but here's a snippet extracted from the aforementioned that you may care to adjust (you can see the extensive comments in the original):

\clubpenalty=9996
\widowpenalty=9999
\brokenpenalty=4991
\predisplaypenalty=10000
\postdisplaypenalty=1549
\displaywidowpenalty=1602

Personally, I tend to avoid this TinXering with Plain TeX internals; although I don't know how to do it specifically for newlfm, I'd probably opt for adjusting the textheight on a case-by-case basis, as a final tidy-up before publishing.