1) Think about what you need from the hosting company both short and long term requirements.
2) Do your research, don't just sign-up to a web hosting company because they are offering a good deal.
3) Draw up a comparison list of what each mambo hosting provider can offer based on support, company history, functionality ( MySQL, PHP, CGI, SSL ) bandwidth, web space, free domain, email and so on .
4) look at the web hosting reviews from real people who use the hosting providers already.
5) Look at how much it will cost you to purchase additional web space, bandwidth or purchase an additional mysql database.
6) Look at what on site security facilities are in place
7) Speak to the developer who is creating your mambo web site and discuss what additional functionality is required. You may need to purchase SSL for a secure  online shopping facility.
8) Read your web host's AUP (Acceptable Use Policy), and their TOS (Terms Of Service), plus their SLA (Service Level Agreement). Make certain that you understand and agree with your new web host's policies, and that your website does not break any of your new web host's rules and regulations.
8) Most importantly review the requirements for hosting mambo