Usually I rely on kino and eye contact to turn up sexual tension when i'm gaming, but i'm having difficulty finding something to do the trick in text based interactions.
By this I mean online messages, online messaging, texts, love letters......

Give a bro a hand?

I don't bother acting sexual in texts. What's the point? Its not like I can turn her on there and then and start ripping clothes off and It runs the risk of coming across too strong too soon and scaring them off. My personal style is under the radar 'nice' guy never see me coming until they are 'cuming' :p

Just some advice but I know you want something you can actually use so:

'I'm not sure who you're boyfriend is but he clearly hasn't been spanking you enough'

Idk about you man, but Ive been able to get girls moderately horny via aim and other chat messaging. Then once that has happened ive gotten them VERY horny on the phone. Depending on how slutty the girl is/ how much attraction you have/ and how much comfort you have will decide how far you can take the sexual talk. One thing I like to do to introduce a sexual vibe into a conversation is getting the girl to say something, anything superficial about herself that you dont give a shit about. Ex:
PUA: "whats your fav color?"
HB: "green"
PUA "ok good, if you said red I would have kicked your butt........as tiny as it may be"
HB: "my butt isnt tiny!"
PUA: "I'll be the judge of that" OR "I dont believe you, im going to bring my microscope with me just in case"

Like I said, depending on the level of attraction, comfort, and her sluttyness level that conversation has a capability of evolving sexually. If it does, I like to throw in a couple sexual lines where I see fit (girls always respond well to these 2 lines)
line one (mystery) "ive eaten girls like you for breakfast" they always seem to take that as a challenge and press the sexual conversation forward. Line 2 (a line from my former wingman evicarta) "please, give me 15 minutes and I'll have you squirting like a hose" This one also works well because it creates the frame of a CHALLENGE so that they can talk dirty back to you without sounding or feeling too sluty, afterall, your the one who challenged her lol. Her rationalization process will tell her that she's simply defending herself rather than "talking dirty" to you.

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There are 3 type of players, those who make it happen, those who watch it happen, and those who wonder what just happend. Which one are you?

both posts above have good points. I know I've turned things sexual and it was very successful at building sexual tension, but I can't think of anything off the top of my head.

One thing I will say is that sometimes a women will make a sexual innuendo or some other seemingly naughty comment. Never let her get away with that. Always give her a hard time about it. And keep your eyes out for a moment when you can respond "oh that's the stuff your into...kinky." I use that line a lot but you have to wait for the moment.

I think the line about spanking is gold.

If things are neutral and you want a way to turn them sexual without it being too obvious you can play the "I like game" where you just take it in turns to name things you like.

Start with something funny, i.e. I like jelly, then something personal i.e I like to run late at night when there's no one else around then something suggestive i.e I like
to maintain eye contact longer than normal with people I'm attracted to and then change gear and mention something sexual you like that would clearly benefit her.

If not you can always play the 3 Q's game, all questions must be answered by both sides. Or shorten this and just say "I have a question for you if you promise to answer honestly" and follow it with an erotic question that again suggests a benefit for her. Sometimes the shock of the erotica is enough to get her ready and willing to talk about sex.

Nice one fellas. I'm on the case.