The biggest key to LLM success is loans-in.
Make use of
every loan you possibly can, and try to set them for 0% of wages. The only thing better than a free transfer is a player who is absolutely free!

(If you have a feeder club, don't wait for them to offer their Reserves to you, go bid for them! Other than that, follow the "Offered for Loan" list carefully.)
Keep your wage bill low low low. That means, as
soon as you've identified a player as deadwood, he's straight on the transfer list. You may need to cut his asking price to ZERO, that's okay, as long as you can get his wages off the books. If nobody'll buy 'em, find out how much it'll cost to offer them Mutual Termination.
You can't afford to have a Reserve and Under-18 team. Don't even bother. Grey players will fill out the ranks, that's what they're there for.
Since your goal is to promote, don't lock up players to long-term contracts unless you think they can do the job at the next division up.
Think of the club as a business. You aren't in the business of collecting Premiership-quality players (
yet!). So, like any other business, you need to buy low, and sell high. When an offer comes in that you "can't refuse", don't refuse it!

Remember, that value is going to be different than it is as a Premiership club - as a Conference club, I found a £250,000 offer for my best player to be well worth taking.
Lastly, be patient - as anybody who's played dafuge's challenge will tell you, success isn't going to happen overnight.