Signature campaign, social media campaign and other types of campaign. There are also some agencies (companies) that are offering marketing for ICO's but it cost you money so it's up on how much the budget for the marketing. Newsletters, publication of articles on news websites and price tracker banners are some of the techniques.
Do you know where can I find some statistics about signature campaign results? All the Bounty Campaigns put a lot of stakes on Signature, but never saw some metrics