Postby Javizy » August 26th, 2010 9:46 pm
o-negai shimasu is just a humble form of the verb 'negau' which has come to be used idiomatically to mean 'please', and kudasai is the imperative form of the verb 'kudasaru', the humble equivalent of 'kureru'.
As you can see, they're both already very polite forms, and having 'verb-imperative + verb' would make no sense, so you have to have one or the other. They're not always interchangeable though. It's possible to make politer (keigo) requests using other structures such as 'o + masu-stem + kudasai', depending on what exactly you're trying to say.
As for informal forms, like I mentioned above, 'kureru' is the neutral equivalent of 'kudasaru', and it's imperative form is 'kure'. This is very informal though, and you're better off sticking with iaai's suggestion of the te-form until you understand more about politeness levels. With o-negai shimasu, you can just drop the shimasu, but it sounds kind of funny in informal contexts anyway.