What is the best sales strategy for startup companies? and what challenges do they face?

BEST ONE IS TO HIRE PROFESSIONAL SALES MANAGER TO IMPLEMENT THE STRATEGY ...........   

GOLDEN RULE  ........  FOCUS ON PROSPECTIVE CUSTOMERS LIST & GIVE STRONG SALES FOLLOW UPS TO ESTABLISH YOUR SALES PLAN......... GOOD LUCK

It depends on the industry in which the firm performs . 

An FMCG start up strategy is not applicable to automotive or building industries .

But the only advice for a start up business in sales is : Focus only in "one customer segment " , analyze it very well and give them better options than competitors . Have a uniqueness ( in product specifications , service style , brand perception etc )