This is REALLY bad for me in Cocoa/Cocoa Touch apps. Objective-C is so verbose and writing classes is so "mechanically cumbersome" (having to write .h and .m files and duplicate stuff, for example) that I sometimes lose interest before I ever really get off the ground.
The trick I use to beat this tendency is to totally eschew good software engineering principles at first. Typically all of my code lives in main.m (and I usually start out in CodeRunner rather than XCode). Interfaces are done sloppily in Interface Builder or entirely in code. This really helps me get started.
Before long, of course, I have this unwieldy spaghettified mess of a single source file. By that time, though, I have some stuff WORKING, so doing some refactoring into a good architecture for the code I have while keeping the working stuff working is kind of fun.