I've rarely had it be much of an issue with my warlock - usually you curse an enemy and focus on them till they drop anyway. Plus with moving to keep shadow defense bonus, if I want to target somebody I usually can.
Yup, Rod of Corruption and Quickcurse Rod are top choices.
You also want to try and be the closest to an enemy, without putting yourself in too much danger, so that you can take advantage of the +1 to hit (and potentially many more benefits via feats) from Prime Shot.
Mobility is key and the Warlock is very heavily invested in the action economy. It would frequently take me 3 rounds or more to work up for a proper shot at multiple cursed targets, with Cursebite and Cursegrind.
On the mobility front, my Dwarven Starlock gets value out of his Ethereal Slide power every time it's used. And his 6th level power, Mirror Darkly (HoS p91), is doubly useful- first it creates a Shade that can serve as a Point of origin for your powers, then, at the end of your turn, you can teleport to one of the squares used to occupy. This means it gives you different LoS on targets and extends your range by 5, then you move several squares away. Stick & run!