The Key to poker straight hair: Get the cuticle of the hair as flat as you can at every stage.
Start in the shower. Ditch the store-bought shampoos, as mentioned they are full of silicones/waxes, when there's a build up this on the hair shaft the heat from straighteners can't penetrate the hair cuticle as much as it should, thus unable to give the best result.
Choose shampoos & conditioners created for straightening like Tigi Control Freak products. They leave the hair in a condition where it's more pliable and any styling you do will take easier. Days when you don't plan to straighten your hair use a deep-conditioning treatment (say, at least once a week), or else your hair will become brittle & resemble straw -- definately not giving you a glossy straight finish!
Thirdly, after blotting the majority of water out of your hair (don't brutally rub the hair because it ruffles the cuticles up), use a small amount of straightening balm (not too much, don't want the hair to look dank!)
Then blow dry hair in sections. And i mean PROPERLY blow dry, not blast the hair about in all directions... Watch how your hairdresser works next time you're in there.
Starting at the back (you might need some assistance the first few times), take a large round bristle brush (pref natural hair as it glides flat against the cuticle), and 'pull' quite firmly on the hair, gliding the hairdryer down the hairshaft with a concentrater nozzle (flatter the better). Use that technique all over in sections, make sure it is fully dry.
If you've done a good job with the blow-dry, half your work is already done.
Now heat up your straighteners, ensuring they're as hot as your hair can withstand (less heat for chemically processed hair remember!) section the hair again.
Starting at the back, grab some hairspray, spray down the section of hair, working quickly, comb through to distribute evenly, then run the straighteners down it.
Depending on the type of hairspray you used you'll be left with a sort of crisp hardened effect. You can either comb it through, brush it out, or leave as it is, depending on the look you want to achieve, but providing you've run the straighteners down the hair in an even, straight, consistant way, you should now be left with a poker straight section of hair which falls into sectional pieces, and shouldn't get effected by humid conditions. This is the Hairdressing technique called 'Blades' of pin-straight hair.
Good luck!
^^^ Not my advice but found on hair forums elsewhere.