What is the best sales tip useful for any sales professional?

You must have full info about your product & its competitors because the customer is always smart & clever.

The best advice that there should be a mix marketing and sales promotion More importantly, sales promotion through promotional mix And characteristically announced and raise the quality and reduce the cost and intensify sources be found for the client or the final consumer in arabic بالعربيه افضل نصيحه ان يكون هناك مزيج تسويقي وترويجي للمبيعات والاهم تنشيط المبيعات من خلال المزيج الترويجي والاعلان عنها بشكل مميز ورفع جودتها وتخفيض تكلفتها وتكثيف مصادر ايجادها بالنسبه للعميل او المستهلك النهائي

to be Smart and Market intelligent

all the other answers count for nothing if you do not understand the product you are trying to sell ! know your product, know your market!

be persistent with regular progress check

Determine your targeted client's value proposition in order to engage his interest/ loyalty toward your product/ service.

Always keeps a smile in your face.