I picked up this little gem from The English Woodwoorker. When face planing, use a thin batten with a notch cut into it to hold the corner of your work piece. The batten should be held diagonally by a clamp or hold fast. This will secure your work piece when you plane diagonally and cross grain. No need for an end vise.
Tip: it helps to have a bench dog engage the piece at the opposite corner. This will act as a pivot point to engage the batten.