Note: A lot of this is more general than just discussion of Classic. So, some examples may use variants with which some players are unfamiliar.

Recently, I've realized that in the spring of the first year, a lot of players will send a unit into an adjacent SC. Then in the fall, that unit will often do NOTHING. Why??? You could be defining the gameplay in this crucial first year. Maybe that unit doesn't actually have something worth doing in the first Fall. Maybe there really isn't anything worth doing. In that case, why did you rush there in the spring? Did you just want to hurry up and wait? THAT IS POINTLESS!!!!!

The worst-case scenario is when, in the spring, while your unit rushes (unnecessarily) to an SC and your opponent moves to threaten that unit's original position. You then send your unit back to defend but your opponent doesn't attack so you lose the SC you had been sitting in. Or, on some maps (not classic) your opponent takes the neutral from you.

For example, consider South Australia in Mate Against Mate. (http://vdiplomacy.net/variants.php?variantID=37 There are actually tonnes of possible examples on this map.) South Oz's army at Alice Springs could slip into CP in the spring. BUT WHY????? There is so much else that it could be doing. Imagine that AS moves to CP while Broken Hill moves to Eyre. Then, in the fall, CP covers AS while Eyre sneaks into... CP!!!

As an alternative, AS could have attempted a bounce with a possible attacker. Matilda Track, Great Sandy Desert, Tennant Creek or Eyre are all great choices. By moving to one of those, you are stopping the worst-case scenario. And, if the expected bounce doesn't happen, then you have a unit next to someone else's home SC and they are the ones in the awkward position!

Now, someone is going to say "But if you fail the bounce, you can't take the SC!" WRONG. (Sometimes.) In the above example, AS can move to Eyre! Then, they are stopping a possible attack (if it comes) while staying in position to take CP! So, you have two birds covered with one stone!

And this isn't the only situation where a unit can do more than just beeline to a destination neutral SC. Even in the case where the trip will take a full year, more can be achieved. For example, in classic, Russia has two routes to get St.P(sc) to Sweden. They can go through Finland or Gulf of Bothnia. Here, Gulf of Bothnia is generally the better choice because GoB can then move somewhere like Baltic Sea if Germany seems aggressive. Finland on the other hand, would have to go through GoB to get to Baltic. Another example (I would say a better one even) is this game of WWIV (http://vdiplomacy.net/board.php?gameID=5649#gamePanel) where I am Congo. In the first year, my fleet at IN can go to LUB or STH. Through negotiations, South Africa lets me have LUB and I let Brazil have STH. But, I move to ANB. Why??? By moving to ANB I don't risk a bounce, and have both possibilities. If South Africa does an immediate stab by going to NAM or SAT, I can then try to negotiate for STH instead of ANB. Or, if Brazil seems content to give STH to Nigeria (which I really don't want) I can then oppose this move. The same logic is shown by Nigeria moving to ESA rather than to IVO in order to get to DAK. In both cases players increase their possible influence while accomplishing the same goal.

A similar principle applies throughout the game, especially in gunboats. EVERY TIME YOU ISSUE A HOLD, YOU'RE WASTING YOUR UNIT!!!! If you have a unit that's holding, think about ANYTHING it could possibly be doing. It's better to issue a support that will only work if your opponent CDs or is incompetent than to do nothing. Because you never know if your opponent might be a moron. And in gunboats, you can always try to signal by doing some sort of move. Hopefully, others will catch on. So, think very carefully before you issue a hold order.

Also, this strategy can be taken to a greater level than just 1901. Take, for example, the Bohemian Crusher move in Classic. Italy gets into position to take Trieste in the Fall by moving to Tyr and Ven. But if he goes for Trieste, what's that unit going to do? If Italy's given away that he's attacking Austria (which he has, by his aggressive moves) Austria will recall troops, so they will end up in a locked up position with Italy only having Trieste, until Russia walks in and takes the rest.

In the Bohemian Crusher, Italy doesn't rush straight for Trieste after moving to Tyr. He moves Tyr-Boh, Ven-Tyr. This, combined with a build from Tunis in Venice, means Italy is almost certain of getting both Trieste and Vienna, which gives them a decent shot at Bud and, later, the Balkans also.
By not 'rushing in', Italy ends up a lot more powerful. Although it gives up on the 1901 build, it makes up for it in 1902, and throughout the game.

This idea of trying to accomplish something extra doesn't always apply. In a press game, you don't need to try unnecessary bounces, because they're likely agreed upon. Then, when someone breaks a DMZ, you're more certain that they're attacking you rather than being a pain. And in a gunboat game, Russia might want to signal to Germany by moving through Finland. An example which seems to break this rule is Austria rushing to Serbia in the Spring. However, this unit generally will do some sort of support move like Albania -Greece or Budapest - Rumania. It is not wasting its move

But, in most cases, some general principles apply:
1) Never issue HOLD orders for your units when they have alternatives. Even if all you want the to do is stay in their present position, always have them doing something when possible.
2) Don't be such a dot grabber that you rush blindly into supply centers whenever they are available. Play both spring and fall phases with an equal amount of enthusiasm. Games are won by the moves you make in the spring that allow you to hold onto the centers you have in the fall.