Practice will get you there but there is an alternative to downtube/seat tube mounted water bottle holders. Touring cyclists often used "feedbags" that are mounted on the inside of the handlebars to one side of the top tube. Here is an example from REI of Revelate Designs Feedbag - if you Google the term handlebar mounted feedbag you'll find lots of less expensive versions, but I'm a big fan of Revelate bags.