For someone who is new to freelancing, what are the best practices in finding ideal clients to sell my services to?

I've been where you are - it kind of sucks :)

I think you need to do a bit of a reset on your personal mindset - Effective agile management is never a "nice to have" - its what keeps things flowing, working, and on time.

To that end - do you know who your ideal client really is?

Let's break down your options (per your question):

Startups - If not funded, move on. You don't need equity, you need revenue. If they are funded it could be a good fit..

Creative Agencies - I think you're giving creative agencies too much credit here - you need to narrow that down. Are you looking at marketing and advertising agencies, or software development shops? The former usually is...lost?... when it comes to Agile, so you'll have a bit of educational work involved, the latter more than likely is already agile by nature. I've also found that sometimes agencies will shy away from outsourcing things like project management (its a control issue).

Solo guys - this all comes down to one question - "Do they have money?"

I'm sure you feel a bit like you're in a rock and a hard place. There are clearly some variables here that I'm not covering, but would in a chat. The bottom line is that you need to get a handle on WHO you want to work with, WHAT you are solving for them, and WHY your services are better than what they are currently doing.

People buy solution, not prevention. With that in mind, you should be able to find a starting point.

If you would like to have a chat, schedule one with me - My team and I do growth strategies for businesses of all sizes, and if we have a chat here on Clarity, and I think I can help, I'd be willing to do an in depth 45 minute session with you for free.

I'm in a similar place right now and one resource I can suggest today is http://expensiveproblem.com/
Positioning yourself in a narrow, focused fashion will get you the clarity you need to pursue the right type of client.

I've found that guest posts are a great way for freelancers to (a) reach potential clients and (b) position themselves as experts. In fact, nearly all of my clients have come from guest posting and even a small number of posts can go a long way for freelancers. Related to point (b), guest post leads are easier to convert since they already view you as an expert.