Depends how many users you need.
If it is up to 100 then Open License is the way forward, which you pay for the license outright and then you have the opportunity to purchase Software Assurance (the ability to upgrade the license to the latest product, usually an annual payment ) With OL you can purchase as little as 5 office licenses to kick it off, it supports all forms of Microsoft products.
If you have more than 100 but less than 250, then Open Value is your best option. Here you purchase licenses and spread the cost over 3 years, then you can just pay annually for SA after that, so for instance you have 125 users and you want Exchange and Windows Server CAL's plus server licenses it works out to around £14,000 PA for the first 3 years, then £3000 for each year after year 3 and you can upgrade to each new version of the license you have SA on.
This is what we have done at work, as over 9 years, it was much cheaper than doing OL. If you are really big, then Enterprise Licensing is your bag, but thats mega money and 000's of licenses, which you really should speak to MS direct or a large reseller.