What should one do to guard against premature ejaculation during sex?

Work on yourself. Premature ejaculation can be difficult to cope with at any age. Things to do to help are reducing performance anxiety( pressure on yourself to perform), sexual position changes to reduce muscle tension( missionary) and lastly positive self image. Feeling confident and relaxed without negative self-judgment will help. If you feel it may be physical, consult a urologist for an evaluation. Good luck.