There are a number of lifestyle factors that can contribute to DNA fragmentation:

Heat. The reason men have a scrotum containing the testes outside the body is that the testes should be about 2 degrees cooler than body temperature for normal sperm production.  In animal experiments where the testes were exposed to temperatures at or above body temperature, results clearly showed that environmental heat to the testes causes sperm DNA fragmentation and is related to infertility.  How does this affect men?  First, your doctor may recommend that when trying to conceive, it is preferable to wear boxers rather than briefs.

Lap top computers.  They generate a huge amount of heat and if used directly on the lap could damage sperm DNA.  Place something between your lap and the computer.

Hot tubs.  Hot tubs are above body temperature and should be avoided during times attempting conception.  Likewise, do not take long, hot baths; use the shower.

Obesity.  Overhanging belly fat will cause a heat trap that can cause sperm DNA damage.  One can at least place an air seat cushion (often used by long distance truckers) on whatever one is sitting on.  Thus, the testis is close to the air cooling cushion.

Diet.  A proper diet of wholesome foods full of vitamins and minerals is sensible when trying to conceive for both women AND men and should be part of your TTC plan.

To mimic elevated testis temperatures caused by clothing and other insulating items, a wool sock was placed over bull testes for 48 hours. This elevated scrotal temperature adversely affected both epididymal and testicular sperm seen by an increased doubling of the %DFI (4% to 8%) within three days and a 5 fold increase by 12 days after the sock removal. Of interest, sperm at 12 days post stress had significant presence of sperm nuclear vacuoles. (Karabinus et al (1997) J Androl 18:549-555.

Mouse body temperature is about 37C. Placing mice into a heated water bath so just the testes are exposed showed that after 1 hr at 38C had minor effects on sperm DNA fragmentation 3 days after the increased temperature had very little effect on increased sperm DNA fragmentation. However, raising the temperature only 2C to 40C caused a more than doubling of %DFI and a more than tripling of %DFI by day 14. Testicular weights of mice under such exposure did not decrease after 35 days; thus, the damage from what may seem as a minor exposure to heat caused very significant sperm DNA fragmentation.
These results from animals and that seen from humans clearly shows the highly negative effect that excess heat to testis/epididymis can have on DNA fragmentation, and likely negative pregnancy outcomes from the damaged sperm.

Sailer et al (1997) J. Andrology 18:294-301