Use another form of birth control until 2 sperm samples show that there is no more sperm in the ejaculate.  This usually takes 3 or 4 months to happen.  I recommend taking in your first sample after 3 months.  The second sample should be taken 1 month after the first one.  If both samples show no sperm, it is safe to have sex without using other forms of contraception.

Pain after a vasectomy is almost always due to any swelling that has occurred.
To prevent swelling:

• Apply ice or a bag of frozen peas for at least 15 minutes three times daily for 3 or 4 days… more often is better, especially on the first day.
  
• Wear tight underwear or a jock strap for 1 week if it feels more comfortable to do so.  Avoid wearing tight pants.
  
• Advil (Ibuprofen) is an anti-inflammatory pill which means that it shrinks swelling.  I recommend using Advil only if you have any discomfort following your surgery.  It does not require a prescription and 400 mg should be taken three times daily for several days.  Taking only a few doesn’t work as well as taking it three times daily.  Don’t use Tylenol - it doesn’t reduce swelling.