I did a fair amount of modern gaming in the 1980s and 90s. Don't think I've been in a moderns battle in 20 years now, so all that kit I collected doesn't even qualify as "modern" anymore, but more like "post war" or "cold war".
I've always struggled to find gamers (and time for gaming). In the modern (cold war and late cold war) era I also had some trouble finding rules I liked.
I played WRG (Wargames Research Group) rules for WW2 and post-war up through about 1980. I finally gave up on the originals. I wandered through Combat Commander, Battlefield Commander, and Challenger (through 3 updates and 5 digests). I had some good and memorable games, but never settled on a ruleset I liked. I had a similar wander on WW2 rules, but finally in the last 15 years I've settled on one ruleset I quite like (Mein Panzer) and one I can repeatedly play and not grouse too much (JagdPanzer). I'm still not settled on a moderns set.
During the '80s and early 90s, as often as not our playing area was a garage floor. Rather expansive, perhaps 10' x 12' or more. We would give it a good sweeping, tape off the boundaries, lay out the terrain, and off we'd go. Lost surprisingly few pieces to step-and-crunch accidents. But it was far too tough on the knees for me to continue that approach.
Gaming since 2000 was mostly on my ping-pong table. Again I put taped boundaries for a game area that is usually smaller than the full table-top, generally measured out at 5' x 8' or 4' x 6'. It's about the limit of reach for me -- I can just get to the middle of the table.
Since I lost my house in the 2008 downturn, I find putting a game together far more difficult, as I no longer have garage space and a ping-pong table available to set up as a gathering place. Poor me.
As with several others here, my preference is 1-to-1 vehicle unit scales (one model = 1 vehicle), and 1-to-squad infantry unit scale. One of my major complaints about WRG was the 1-to-fireteam infantry unit scale. It makes too great of a difference between vehicle game play and infantry game play. 10-12 APCs to a Russian mech company, but 36 stands of Russian infantry, means that the game comes to a screeching halt as soon as the infantry get out of their APCs.
My guiding principal these days is no more than 20 - 25 gaming pieces for each gamer. Seems to be the best way to keep the game flowing. With a consistent unit scale (vehicles and squads) that means typically one reinforced company per player. It works pretty well.
I use hidden movement. I can hardly imagine playing without it anymore. My approach is pretty simple to add on to any ruleset. A paper "chit" is provided for each playing pice (vehicle, squad, whatever). Usually I just slice up a brown paper grocery bag into ~1cm / quarter inch squares, and on one side I write an identifier for the unit (ie: BMP-1 Plt CO). At game time I may also put the gamer's initials on the other side of the chits, if there are several players involved.
The chits are played face-down on the table just like the actual models. You move, you measure, you spot, etc. When a chit is "spotted" as per the rules (whatever ruleset you use), you replace the chit with the model.
I give each gamer a surplus of about 25% blank chits. Gamers can move and play with these blanks as if they were real units, except of course they never actually spot or shoot anything. Keeping up the charade of the blanks is a bit of a learned skill, but once learned even a shadow force of even 25% can lead to substantial uncertainty for opponents.
If the gamers want, they can replace models that are no longer in line-of-site to the enemy with 2 chits -- their real unit and a blank. The moment the two chits go separate directions, the charade is on again.
Players are allowed to react to the chits in any way they choose. Divert resources, adjust orders, call for interdiction area fires, or just ignore them (at your peril). They are the commanders' suspicions, the reports of noises or dust clouds, etc. that drive any CO to distraction.
Once you play with chits, you'll never go back. Really. Not knowing, not having a God's eye view of the enemy's force changes everything in the gaming experience. And it adds very little overhead to the game mechanisms.
The chits have occasionally generated very amusing, but also somewhat realistic, battlefield "friction". For example in one game, while I was advancing my Soviet BTR motor rifle battalion with a company of T-62s in support, my opponent was moving quickly to take up advantageous positions to block my advance with a tank-heavy combat team of M60A3s and Mech Infantry in M113s (with a ITVs in support). He came into a town in column, and then split his force to take positions with his tanks on some high ground, with edge-of-town flanks covered by the mech infantry. About two turns later he suddenly got frantic. "Wait a minute... what the?.... Where did they go?" It turns out he had forgotten which chits were his blanks and which were his units, and had moved a whole platoon of actual mech infantry off to a bluffing position, while a vital flank approach was being guarded by blanks. There was no reason he could not have checked under the chits as he disbursed his formations, but he just didn't. And the newly minted butter-bar for the 2nd platoon evidently didn't know his left from his right, or maybe just wasn't confident enough to ask for clarification of his orders: "And when you get to point Bravo, what do you do? " "I go left, sir...?" "Right!" "... uh ...yes sir!"
For terrain I have learned the following approach. I make terrain heights out of corrugated cardboard. Just cutting shapes with a box-cutter out of various boxes that stuff comes in to our house. For years, whether it was TVs, computers, appliances, whatever we got, if it came in a box, I cut it up. Some were sequences of of similar shapes but declining sizes, others just random shapes. Ticky-tack (sticky but removable "stuff" bought at the crafts store) will hold them in place on the table. Then a game cloth is laid over them again with ticky-tack to hold it down in multiple locations. Better not to use felt ... just a cloth of appropriate base color (green or tan). Then pastels to draw the roads, water courses, dirt / mud fields, highlighted slopes, etc. Then my terrain pieces like buildings, houses, bridges, trees, hedges, walls, etc. Can be sparse or crowded, and the area is large enough to be both -- some denser terrain and some open portions.
At least that's my approach. Your mileage may vary.