I've just started out a few weeks ago, here are a couple of tips that I got:
Do the Blake Snell Program - dont necessarly get Snell (as you have to lock in players), but grind and get two $5-6K diamonds and one that was about $25K, although the prices could be a bit lower now, than they were a week ago.
Conquest - do the small maps. I did the 4th inning map in a night and got a ton of packs, stubs etc. - I'm now going to start working through the other small maps for the same reason.
Showdowns - I have only done one, but again it was something I could knock off in a night. I don't fully understand the showdowns, but they will give you points towards the programs, which I think is the big draw to them. Don't forget that you can adjust your line up after every game to put in your upgraded players (I didn't realize this until the end, and made a couple of bad swaps because I couldn't remember which players I had gotten in prior rounds)
Player Evolution - I took CarGo and got the card in an afternoon. The moments and missions aren't that difficult.
Work on the other player programs. I havent started here, but there are 3-4 guys that I will go after. i assume that this is also going to help me get points towards those programs which should get me more loot!
Anyways, that's what I have learned from the forum and the game. Enjoy and don't buy packs!