Make sure the introductory period for any card is at least 12 months. Jonathan, an anonymous personal finance blogger who opens his financial life to the public at Mymoneyblog. With Acorns, you open a free account and link your credit card and bank account. While the above tip is great, it does involve some work on your end and it also requires you to be disciplined to actually invest the money. Continue Reading. What is the catch you ask?