So in SFM the phantom flexes are very bugged... Bugged to the point where if you don't set them properly you won't be able to change the flexes on that model for your entire SFM session (even in a new scene).

Start by aiming your camera at the clothes, for some reason you need to look at it for this to work.

If the clothes's flex doesn't work first start by going to the elements viewer by right clicking the model, then going to Show in Element Viewer > Model
http://i.imgur.com/6233vyK.png

Open up globalFlexControllers and flexnames
http://i.imgur.com/6MBHstV.png

Right click and copy the respective flexes in globalFlexControllers until it matches the same order as it does in the flexnames list. Be sure to NOT delete the original ones.
http://i.imgur.com/obsx6xn.png

Go through and set the flexes you want to be set to 1 in globalFlexControllers, except for the first one. Changing one of them should apply the flex, but we're not finished yet.
http://i.imgur.com/hwjcS3V.png

Right click and copy one of the flexes you've set to 1.
http://i.imgur.com/4aBZ9Dg.png

Then right click the first flex in the globalFlexControllers that you want to change, and do Paste Special > Paste as Reference.
http://i.imgur.com/I6VL1Zi.png

Viola! The flex should be applied and hopefully should be safe when opening and closing the session. If it doesn't work... Well... You're probably best off setting spawning and setting the flex before rendering your poster/animation