There are two ways of haring a trail, Pre-lay (exactly what it sounds like) and Live, where the hares are given a brief headstart and then chased down by the pack.
Pre-lay:
Most of these apply to live haring to some extent or another, but are most critical if you're laying a trail the day before (or earlier, or later)
If your trail doubles back near itself, be extremely careful about where you place checks, and how well you label your turns. It's entirely possible that the pack will bomb past a turn arrow, right into a leg of trail that by rights should be 3 miles later on. Expect to down-down, especially if the pack misses a drink check as a result.
CHECK. THE. WEATHER. If the weather report is calling for rain, flour works better than chalk, but can still get washed away if trail is on the ground for a long time before the start, or if the rain is coming down heavily. Try to put your marks under awnings, trees, in the leeward side of buildings (the side sheltered from the wind) to make it last longer. Still, expect to hare live, or at least to shanghai one of the FRB's in the pack into haring.
If you're running a night hash, make absolutely sure you do NOT use dark chalk. White, Neon Green, Yellow, Orange, Pink, are all acceptable colors. Blue, grey, dark red are not. Also, try to leave at least one in every few marks under or near a light source. Bamboozling the pack is fun, but if they're looking for each mark like a lost contact, you will have an extremely unhappy pack of wanks to deal with at the on-in. Assuming anyone finds it
Live Haring:
It's much more easy to cover the same territory when live haring, because you can return to a marked trail after the pack has already run through. Still, use a distinct color of chalk for each part of the trail, and maybe a special mark. The average harrier doesn't have a lot going on upstairs even before the prelube.
If co-haring, make sure you both have phones. It's easy to get separated laying false trails, and with a pack of baying drunks behind you, you don't have a lot of time to go running around finding each other, and shouting is pretty much out of the question.
Strategy is your best friend. If there are ever 4 or more ways to turn (not counting the way you came, leave a check. Make sure checks turn uphill often enough that the FRBs tire themselves out running uphill at each one. If there's rough shiggy, go straight through. Security guards generally aren't fast enough to deal with one or two hares who surprise them and are half gone before they stand up, but they'll be ready when the pack gets there : )