I'm the only developer at a small company. I've slowly moved into development here; until ~4 months ago 50-75% of my time was spent on operations. Now, 50-75% of my time is spent on development, with the rest split between operations and various IT stuff. I regularly end up working 50+ hours a week.
I inherited some rather poorly-written applications (they were previously maintained by two people) that much of the business relies upon. Keeping these up and running, working on new, smaller applications, and my other responsibilities already take up all my time.
In order to be scalable, the existing software needs significant refactoring and additional functionality. I haven't had the pleasure to work on properly written or architected software before. The complexity of this task is well beyond anything I've done before (this is my first job out of college.) I know there's a feverish devotion to self-learning/learning by doing among many here, but this is so beyond my expertise that I wouldn't be doing my employer or myself any favors trying to tackle it alone.
I've been very direct about my inexperience, and in the past have mentioned that hiring another, more experienced developer will probably be necessary...if anything, just for the amount of time required for anyone to do the work as we grow and have more software to develop and maintain. I know that I would greatly benefit from hiring another developer; having someone to learn from and bounce ideas off of would be great. StackOverflow is great for determining approaches to individual coding problems or concepts, but is no replacement for discussions on a wider or more significant scale specific to a certain business domain. When mentioning hiring another developer in casual conversation recently, they didn't seem to think it was that important or necessary.
tl;dr: Current patch jobs and other responsibilities already take up all my time at work, work on existing applications that needs to be done is beyond my skillset, little chance of me having any time to work on new products that are being planned. Employer initially seems reluctant about hiring another developer.
How can I "sell" hiring another developer without sounding like I'm lazy or incompetent (I'd like to think I'm neither!)?
edit: Just wanted to clarify that I'm in no way interested in taking any kind of hostile action to prove a point (i.e. taking a vacation to show them they'd be screwed if I wasn't around.) I'm pretty content working here and consider myself to be fairly compensated, even figuring in the overtime, which is why I'm nowhere near considering a new job yet. That said, I accepted the 'no more overtime' answer - even if I don't mind working over too much I'm not doing anyone any favors by doing so (prone to more errors, wear myself out) and it's not really tenable in the short term much less the long term. I'll be stressing this when discussing the matter with my supervisor, and will probably suggest hiring a contractor part-time as an initial approach that's more financially palatable. Thanks for all the great answers.