Best way to protect hair you don't want to dye when upside-down dip-dying

i don't have anything constructive to say, but i'm actually glad you asked this-- i've been wondering too.  i've seen tutorials of it done on youtube (some of the videos called reverse-ombre are what you're talking about, but only some of them!)... but it seems pretty impossible to do well without someone to help you.

If it's a pure semi, putting conditioner over the ends will stop the dye from taking.  If you apply the conditioner a bit haphazardly with your hands, you should end up with a soft line.

I do this all the time with semis, seriously.
I grab a conditioner or hair mask rich in silicone and cover the parts i don't want to get stained by the dye and apply the shade i want on the roots and lenghts, trying not to reach where i put the conditioner, leaving a small gap in between those sections because the colour on top will stain that gap anyways when you take it off. Leave it on for as long as you please and rinse it. Bam! It's done~

It always works for me, even on crazy ombre experiments like this one (4 different colours!)

How long is your hair?

If it is long like @pastina's, I would coat your ends in a thick, silicone based conditioner and go upwards with it a bit where you want the dye and bleached hair to blend. I have a couple friends that make mixes out of thick silicone based conditioners and Vaseline too, I've never tried it that way though because of how hard it can be to get Vaseline out of the hair, but it seems to work for them just great.
For more protection, I would wrap your ends in cling film and secure it with a hair tie. If you have someone to help you it would be a lot easier, but what you can do when you are rinsing out the dye is rinse out one section at a time, holding the ends upwards so the dye doesn't run on them. Like, if you are leaning your head over the sink or tub so you're face down, hold one section at a time out and the ends towards the ceiling, so the section makes a U shape. Hope that makes sense!

If you have medium length hair, I would coat your ends in again, a thick, silicone based conditioner (or a mixture of it with Vaseline.) Still though, if you can rinse it in a small section at a time, holding the ends up, you'll be protecting them even more from the dye.

This is a much bigger response than I was expecting, so cheers everyone! @katiesiepierski, my hair is about shoulder-length atm, so I'll have a go with some siliconey conditioner and perhaps tie up some of it in clingfilm, it's not that thick so it probably won't be too much of a faff. Thanks so much @Nerdiemonster for showing me what you do with your hair! I genuinely thought I was pushing it with this idea and that it wasn't possible, so it's great to know that it can be done. 🙂 My next day off is Tuesday and my boyfriend is visiting so I may get him to help me... although I don't see that ending well.

lol-- when my husband and i were in school (and not yet married!) we used to dye each other's hair all the time!  honestly, i bet your boyfriend will be so uneasy with the looming danger of ruining your hair, that he'll end up being super careful, and doing a bang-up job.

I'd love to see the results! Please update us with pictures when you do it!
And i'm pretty sure your boyfriend will do a great job as Pastina said, he'll be super careful not to ruin your hair 😀

I agree, I've had my fiancé do my hair before and he did a GREAT job! I was really impressed! He was super careful, he knows how I am about my hair lol, and it looked great after. I'm sure he will do a great job 🙂

Thankfully we didn't have time whilst my bf was visiting for him to help. I say thankfully because I did it last night and it looks TERRIBLE, the Superdrug dyes really haven't taken well over the bleach (I tried Warm Copper Gold and True Chestnut, the latter twice) and I look like an anaemic flamingo. I work in a zoo so it's not a good look.

Thankfully I have three days off now, but I'm supposed to be visiting my bf (he lives a couple of hours away) so I'm not sure how to fix this! I'm thinking I may just have to stick a semi black on it, but keeping the ends white. The conditioner thing DID work, which is a relief (thanks so much for the advice!) but I'm a bit worried about how it'll fare if I dye the rest black. I have about half a pot of Directions Ebony but it's about six years old, does Directions go off? I know that will fade to green but what if I mix it with Adore Copper Brown? I'm looking into buying a newer pot of Directions Ebony because I understand that it now fades to red instead of green, is that correct? Gahhhh I've really got myself into a mess, thankfully I was hiding in a cupboard for most of today counting stock, but it really looks so awful! 🙁

Semi dyes will last for years as long as they are stored in a cool, dry place.

I hate to say it, but it's possible that your hair may be too damaged to hold the colour.  Strand test the Ebony over what you currently have, and see what it looks like.

I did a silly thing and stuck the Ebony on. It turned my whole head green, even the bits that had loads of conditioner on. Then my mum decided enough was enough and bought me a permanent brown box dye, which didn't take properly and I had a reaction to the PPD! >.< Very sore, itchy face. I'm trying to get rid of the green with whatever red dyes I have lying around, but I've run out of Adore Copper Brown after only managing to apply a bit to the front. I have my hair tied up for work but looks like it's the hairdressers for me. 🙁 Oh well, I had a good run. Thanks everyone for your help!