All Classes
Do Winston's Fossil Dig-up quest. The 30K mesos obtained are a considerable amount for the level (21).
Do the quest Alex the Runaway Kid. The level 25/30 earrings obtained saves you from buying earrings at that level.
Do the quest Fairy's Horn Flute at levels 30-40, as it will give Blue Moon Earrings. This will save you from buying earrings at level 40.
Do the quests "To the New World", "Jane the Alchemist" and "Peace at Eos Tower" at level 40. These quests will give good rewards, boosting your mesos by a large sum. All 3 quests can give a 60% Scroll for Gloves for ATT, which sells for 7-12 million mesos (the actual price depends on the server you are on)
Do the quest Terminating the Dark Force, which is the final sequel to the quest The Ore of Dark Crystal. Although the dark crystal ores might be pricey, they are worth the overall scrolls.
Do Party Quests. Especially for Ludibrium Party Quest, because you may recieve great prizes, such as Capes, Earrings and Scrolls.
Do keep your solid horns or foxtails or anything that can be used for the exhange quest. people will buy them at a higher price as compared to the NPC.
Do keep your money in the bank to refrain from buying unnecessary things.
Do not beg for mesos, it gives people the impression that you are a poor maple stroy mesos player. Some players will get very annoyed when asked for mesos and you might get a defame instead.
Do not sell your ores to NPCs. Instead find someone on the free market and sell it. 100/150 mesos for an ore isn't worth it. Neither should you sell those 1-meso items, chances are, they are worth decent amounts in the market (eg. Omoks, Monster Cards and Scrolls).