Weapons

As soon as you have enough money, buy a strong or very strong weapon (reliability = strong or very strong) with a fairly high strength (+6 or more) and use it as your main weapon.

Don't buy a very high strength (+10 or more) weapon that is very fragile. It will break a lot and you will have to spend a lot of money repairing it.

Sell your weapons, armors and nets in the shop the last day of the month. You are going to lose them when the new quest starts, anyway.

Armours

Buy one or two very good armors as soon as you have enough money. That will save you a lot of money on health potions.

Don't buy too many, they don't break and you cannot change armors while in the dungeon.

Sell your armors the last day of the month.

Potions.

If you finish the day with 3 lifes then maybe you have used too many health potions.

If you have to spend a lot of health potions, maybe you should think about increasing your defense skills and/or buying some armoring.

If you can afford it, don't play one dungeon a day, save them and play 3 or 4 in one day (e.g. the weekend) Buy potions of learning +2 and activate one when you first find a monster, play 3 or 4 dungeons with an active potion of learning +2 and obtain a lot of experience points with just one potion of learning +2. You can also use the potions of attack and defense you find in the dungeons. Remember, they are not additive, you can increase your attack or defense by one regardless of how many potions you use. Same with magic potions.

Buy health potions when the +1 ones cost less than 100 gold pieces each at the shop. If you have a surplus of health potions because you find a lot in the dungeons, then sell them at the market. The last day of the month is a very bad day to sell potions.

Spells

Focus in one flavour of magic, don't try to master the four of them.

Buy only normal or +1 spells, the rest, (+2 and +3) are way too expensive.

You can sell spells back to the shop, so don't buy too many, and buy only when they are cheap at the shop

Other tips

If you can, play the temple and the treasure hunt every day, it only takes one minute and you will get some gold.

If you aim to level 100, don't forget to raise your ruse, so you will be able to capture level 100 monsters and buy them at high price. If you don't raise your ruse skill, you won't be able to capture level 100 monsters.

This website has maps, use them. Specially when you are playing dungeons with a potion of learning +2. You won't get lost, you will find all the keys and chest quicker and you will be able to complete more dungeons with one potion of learning +2.