Be sure to allow people to join your mailing list from the homepage.  If your shopping cart doesn't allow this, there are programs and companies online that handle mailing lists.  Sometimes people will join your mailing list but not make a purchase from you that same day, only to come back and make a purchase after receiving your sales email.  If you don't have their email address to email them, they may never come back. 

 

I suggest offering some sort of small giveaway to entice people to signup for your mailing list.  Like, "Subscribe to our sales mailing list and be eligible for a free 'whatever'."  You could do this every month but it is most effective obviously during your more busy months.  You'll get more subscribers and still have to give out only one prize.  Consider it as an advertising cost.  Note:  I've never had great results with this idea, but at times it's been ok.  It doesn't cost much so it's worth a shot with any business.