Getting content done for a game (and deciding when to get content done) is one of the things I find trickiest these days. It's definitely possible to get trapped into using placeholder art all the time because you don't want to waste any time on content that'll get thrown out. However it's also pretty unhelpful - code will go through lots of iterations before it reaches something you're happy with for the final game, and content shouldn't be any different. Much like code you start with the basics, then throw things out that don't work and refine that which does. And often just the act of doing it wrong the first time will teach you a lot about how you need to do it properly the next time.
It's essential I think to do mock up image before starting on "proper" art. This worked really well for Snowman Village
and Rescue Squad
. It lets you get a lot of the basics down without spending a lot of time doing individual sprites. Focus on the colour palette, the relative sizes of objects, and the overal style (angular, vector, cartoony, pixelated, realistic, etc.). Maybe create a few variations and see which ones you like. It's also a good point to see what's practical and what isn't - simplify the bits which are too complicated to knock out consistently and you'll make your life much easier.
What I did with Snowman Village was stay entirely with a wireframe representation (basically just the collision bounds drawn in different colours) until I thought it was fun. Then I could actually start adding in art and proper graphics. After that point you can iterate your gameplay and content at the same time (adding new items/enemies/stuff with associated artwork as needed).