Ginny/Luna Post-HBP: The Harry Problem
Getting Ginny and Luna together prior to HBP tended not to take into account Ginny's boyfriends. It was easy to ignore the fact that she was dating because since it was never really 'on the page', only referred to, we could pretend either that it wasn't happening or that it was an elaborate cover-up for all the hot lesbian sex that Ginny and Luna were having. After HBP, it's not so simple.
We can't ignore a canon relationship when it is played out in front of us with all the intricacies of why they're together. Admittedly a considerable portion of the justification of Harry/Ginny was several pages of Harry thinking 'but she's my best friend's sister' but that still can't change the fact that Ginny admits she has, and always has had, feelings for Harry. So how do we 'ship Ginny/Luna in light of that fact.
Ignore it. This isn't my final answer but let's consider it first since it's probably the easiest. We can ignore the whole thing either by writing about a time and place further down the line when Ginny and Harry are no longer together or by, well, ignoring it. Writing in an alternate universe and pretending canon didn't happen because we all know that JKR meant for Ginny to go out with Luna, it just wasn't allowed by the publishers (NB: This is a joke. I am not implying that JKR really supports G/L).
Rather than ignoring it, we can instead write future fic. We have the ability to make our G/L relationship more believable by addressing Ginny's prior relationship with Harry. Perhaps by explaining the reason it ended. Often the easiest way of acknowleding a past relationship is to have a shared joke between, in this case, Ginny and Harry which shows that there are no hard feelings. Or Ginny could burn pictures of Harry (or vice versa) and curse the day he was born if you like but that rather suggests she's not over him so then you'd still have to justify the relationship with Luna.
If you still want to write within the timeframe of the books, then you've got some serious explaining to do if you're not going AU. The Harry/Ginny/Luna threesome 'ship is quite popular and that's one way of doing things. Harry's male after all and what man wouldn't like a threesome with two girls? Alternatively, if you're having Ginny cheat on Harry, you need to justify it not just to the reader but to Ginny herself. Perhaps Ginny thinks it doesn't count because Luna is a girl, she certainly doesn't seem to have any lesbian role models to imply that a girl/girl relationship is as valid as a boy/girl one. If Ginny is cheating though, don't forget that Harry's friends will not be supportive of her. Hermione may be Ginny's friend but her loyalty lies with Harry and likewise, Ron would also side with Harry (likely he'd side with Ginny if Harry was the one cheating though).
Failing threesomes and cheating, you need to put an end to Harry/Ginny. It's easy to end the relationship, it's all very Spiderman with 'Harry dumps Ginny to protect her' but that doesn't put an end to their feelings for each other and that is what would stop a believable Ginny/Luna relationship. There's one very obvious way to deal with that: comfort. Ginny could find comfort with Luna but making Luna the rebound girl doesn't look promising for the future. One alternative that has been done well from time to time is to have Ginny get into a love/hate relationship, often with Draco or Pansy, that then moves on to a loving relationship with Luna. It can become clichéd so use it carefully but by having someone else as a rebound, the stage is set for Ginny/Luna to be long term. Of course, if you want to write fic where Ginny is using Luna then that's still valid 'ship fic but it's definitely not fluff and if it's verging on abusive, remember to include warnings in your header.
So despite Ginny and Harry (finally) getting together, we don't all have to give up writing Ginny/Luna (you think it stopped the Harry/Draco writers?) and if anything, it provides us with yet another way to make our fic more believable and that's never a bad thing.