Remember: every creature on a scout is one less creature on your main hero. If you divide your army into pieces, all of the pieces will tend to take higher casualties than if they were all together.

There are three main ways to avoid taking damage or losing creatures to retaliation attacks: have the No Retaliation skill (ex. Hydras, vampires), hit a creature that can't retaliate (ex. Already used its retaliations for the round), or kill the stack entirely with your attack.

There are exceptions and advanced tactics, but concentration of force is simple and effective.

Since you're playing the good campaign, here is some very useful advice: later (after level 2 or 3?), when given the choice, help the dwarves.

What, I've played the game for years and never knew this! I always thought the hero's speed was based on the terrain and their pathfinding skill. Is there somewhere a handy FAQ that lists things like these? I think I read the manual and it wasn't there (or I might be mistaken and only read other HOMAM's manuals). Does this work in the later games as well, by the way?

Thanks for the awesome tip! I usually don't scout and have just few heroes that are the power team, securing the area, but I play only singleplayer where it usually works.

Up till 3. In 4 every unit has their own movement points, and in 5 it's not dependent on unit speed (and combat speed uses a different mechanic altogether). Haven't played 6+.

That's why in HoMM2/3, it's extremely useful for scouts and supply runners to have 1 of the fastest unit, instead of the weakest one (though I usually don't waste level 5-7 units on them). Like the serpent fly, or griffin. And powerful heroes on mop up missions might find it useful to have just one stack of the level 7 creatures (which usually have extremely high speed). It's also why when you can't afford all units, always leave out the slowest ones. And even if you can, it might be useful to leave the slowest ones for castle defense in order to get more speed.

EDIT:
For other less known but useful pieces of trivia try this link:

http://heroes.thelazy.net/wiki/Did_you_know

e.g.
Cavaliers are upgraded to Champions for free at Stables located on the Adventure Map.
A horde building protects those particular creatures in that town from plague, which means that the dwelling with a horde building will produce creatures normally.
Pikemen and Halberdiers are immune to Cavaliers and Champions jousting bonus (+5% damage per hex they cross).
Morale bonus for visiting Temple on day 7 is +2. Also Idol of Fortune gives +1 to both Morale and Luck.

I think of it like this:
-Slowest creature provides base number of move points for the hero for the turn. Feel free to have a nearby hero hold all the slow creatures overnight.
-Logistics provides a multiplier. Stables (HoMM3) provides another bonus.
-Oasis, Rally Flag, etc. provide some extra move points.
-Moving over rough terrain costs extra move points per tile. Some terrains cost more than others (Snow and Swamp both being pretty bad, while rough terrain is worse than grass but not as bad as snow). Roads cost less move per tile than grasslands; cobblestone road allows faster movement than dirt road.
-Pathfinding reduces/negates the move penalty for rough terrain.
-At least in HoMM3, moving on a diagonal costs slightly more than moving straight horizontal/vertical. I think this also applies to picking up resources; if you are walking past a pile of gold, then grab it when it is orthogonal, not diagonal.

At least in HoMM3, if your army is moving over their home terrain (or was it if you have at least one stack for that terrain, or was it if the majority of your army calls it home terrain? I don't remember), then you don't suffer the move penalty for that terrain. So a hero leading Tower troops has no problem moving over Snow.

I forget which details apply to which games. I remember one of them actually displays larger/longer arrows for the path when the tile costs extra movement. This might have been HoMM4?

Scouts are great for uncovering map so that your primary hero knows where to go and not to waste time going. I prefer to stack everything strong on a single hero to kill things with fewer losses, and to be able to tackle stronger enemies.

^Pretty much this, though off-hand I can't think of a Level 4 that I'd place on a scout. If they are worth buying, then they are worth buying for the combat hero.

Oh snap! I didn't know this! I guess it never came up because any time I had unupgraded Cavaliers, I got them from a Castle town, so I already had the Stable move bonus.

Upgraded pegasus. My rampart scouts get centaurs in the beginning, but once I can afford to spare one, pegasus gives them more movement.

For exact calculations about movement:
http://heroes.thelazy.net/wiki/Movement
According to that, speed value 11 gives max base movement, so there is no further benefit of taking faster units after that e.g. no benefit of Archangel (speed 18) over Angel (speed 12).

Also, GreenHilltop, in case you missed it, it's the units at the beginning of the turn that count. So if your hero finishes their turn in a town, move all units except the fastest one to the garrison, and then move them back next day. This is one of the reasons why it's sometimes beneficial to end your turn in town even if you can still move 1 or 2 spaces.

I can see that, though I usually just give them a Centaur Captain and call it good enough. Pegasi need to be en masse to be really effective, considering their glass cannon status.

I have occasionally let extra heroes carry a Thunderbird around, if my primary hero isn't close enough to take it. Though again, Hobgoblins and Wolf Raiders are generally fast enough for my purposes.

Thanks for the replies! For some reason GOG didn't actually alert me to any of these messages.

Ah yes, the single strong hero was my favourite tactic as well, but if I have to have scouts it's easier to split some troops between them so they can get all the resources and treasure they encounter on the way.