1. The "default" ...

   \clubpenalty=10000
   \widowpenalty=10000 
   \displaywidowpenalty=10000

2. % my \keeptogether 
   \def\need{\begingroup\afterassignment\n@@d \dimen@}
   \def\n@@d{\vskip\z@ plus \dimen@ \penalty-60
                     \vskip \z@ plus-\dimen@ \endgroup}
   

   Unfortunately, this grabs only an approximate space, and the exact amount depends on the usual "badness" of a normal break. One way to ensure getting the right space is to eliminate the badness of ordinary breaks:

3. \def\raggedbottom{\topskip 1\topskip plus1000\p@ \r@ggedbottomtrue} 
   \def\need#1{\vskip #1\penalty \z@ \vskip-#1}
   

   Which works pretty well, if you don't mind raggedbottom pages.

4. Finally, there is another method that works very well, as long as you use it between paragraphs: (I.e., not in a \vadjust{}.)

   \def\need#1{\par \penalty-100 \begingroup % preserve \dimen@
     \dimen@\pagegoal \advance\dimen@-\pagetotal % space left
     \ifdim #1>\dimen@ % not enough space left
        \ifdim\dimen@>\z@ \vfil\fi % only do \vfil if some space left on page
        \eject 
     \fi \endgroup}

\enlargesthispage*{2ex}