You can use http://rhaboo.org as a wrapper around localStorage. It stores complex objects but doesn't merely stringify and parse the whole thing like most such libraries do. That's really inefficient if you want to store a lot of data and add to it or change it in small chunks. Also, JSON discards a lot of important stuff like non-numerical properties of arrays.
In rhaboo you can write things like this:
var store = Rhaboo.persistent('Some name');
store.write('count', store.count ? store.count+1 : 1);
var laststamp = store.stamp ? store.stamp.toString() : "never";
store.write('stamp', new Date());
store.write('somethingfancy', {
one: ['man', 'went'],
2: 'mow',
went: [ 2, { mow: ['a', 'meadow' ] }, {} ]
});
store.somethingfancy.went[1].mow.write(1, 'lawn');
console.log( store.somethingfancy.went[1].mow[1] ); //says lawn
BTW, I wrote rhaboo