I think David Mc Mohan's sequence of Demystified books could be about appropriate to smoothly approach string theory on a gentle but nevertheless technical level. However, if you are very serious and plan to do research, this does not replace studying the Polchinski bible and many other "real" textbooks
The demystified books are best read in the following order:
Quantum Field Theory
Before you read the string theory book, it is indispensable that you know some Complex Analysis too.
I like these books because the layout is funny, and they contain nice small grained step by step derivations and good explanations of the most important ideas and concepts you need to know when delfing into a new subject for the first time. The purpose of these books is among other things to make reading "real textbooks" about each topic listed easier.
In addition, to learn what should be studied in what order and find additional resources, Gerard t'Hooft gives some good advice about How to become a good theorist and Waren Siegel has devised a Graduate curiculum for people who want to study high energy theory.