This is a question I am asked on a regular basis from small marketing and sales professionals.
We are a member of the ‘ISMM which is the Institute for Sales & Marketing Management’ of experienced professionals.
The ISMM is specifically for this industry and has been recognised for its work and development worldwide, for more information visit the website on: www.ismm.co.uk
I cannot recommend or endorse any particular association, this is only one of the companies who I have dealt with and am myself a member.
Do your research to confirm which association or membership would best suit your industry and business.