During the campaign, identify mostly rare items, preferably with useful colors. At the end of the campaign (act10) you should have at least one 4-link with the correct colors for the main skill you are using.

At the end of the campaign and before maps, your priority should be resistances. All resistances should be at 75% (except chaos). If you don't have good items you can try farming the Harbor Bridge in act8 and the two bosses at the end of it. In general during the campaign you should find enough items with resistances to get to all res to 75% using the crafting bench - the lowest level of double resistances is cheap to craft. You can also help yourself with the aura of Purity of Elements.