I try to think of the final product and move backwards by determining what ingredients will get me there. If there is a beer that I want to emulate to a certain degree I try to find as much information if nothing else so I have a good idea of how they do it. I have never tried a clone recipe but have definitely used commerical breweries as influences in some of my beers by doing a bit of research and in some cases contacting the brewery directly.
If I am trying to brew a particular style I usually find a good commercial example or award winning recipe to get ideas from but not copy. I did that recently for a Kolsch
Yes, this. ^^^
I occasionally brew a so-called clone recipe or actual recipe from a commercial brewery to the T, but most often I'm not afraid to tweak the recipe to suit my own tastes. Most typically what I'll do for any given style is research a half-dozen award-winning recipes for that style, compile a list of all the ingredients and ranges of amounts that are most commonly used, then use my own personal intuition as to which 4-5 ingredients and amounts seem to be the most appropriate, maybe add another unique ingredient if I feel it will complement the style, then brew it and find out.
Also I love to review recipes and descriptions of old historical beers, imagine how I think they might have tasted, and design a modern recipe around that. The best beer I have ever made was a "historical concept beer", based on some research of what was being grown for ingredients in Wisconsin back in the 1860s. I ended up using chocolate malt, 40% rye malt, Hallertauer hops, local honey, and Kolsch yeast. Yummy yum yum. Love that beer, still have one bottle left for a special occasion, and I need to brew it again soon. It's not an actual historical recipe... but it could have been!
So I look all over the place for inspiration, try to design a recipe backwards based on my imagination of how I think it should taste, brew it up, and occasionally I get lucky and it hits the mark right on. Try to make something unique, but still obviously a beer.
I also like to try adding funky off-center ingredients from time to time, but I also find that in a lot of cases, they either don't really work very well in a beer, or the amounts need serious adjustment for future batches. For the most part, I stick with traditional Reinheitsgebot ingredients, because to me, that's what really makes a great beer in most cases.