In many cases you must obtain prior 'opt-in' consent to send marketing communications, and it is always good practice to use opt-in boxes when collecting contact details for marketing purposes.
You should tell people what methods of marketing communication you are going to use, eg email, text, phone, automated call, post.
You should ask for consent to pass contact details to third parties for marketing, and name or describe those third parties with enough detail to give people an informed choice over marketing.
You should record when and how you obtained consent, and exactly what it covers to ensure you do not inadvertently contact people against their wishes. You can keep a 'suppression list' of people who don't want to receive marketing.
* Direct marketing checklist, ICO
* Consent, in ICO direct marketing guidance
* The DMA Code, Direct Marketing Association website
* What counts as consent?, in Electronic and telephone marketing, ICO Guide to Privacy and Electronic Communications Regulations