a lot of wah pedals have built in boosts when they're switched on, and there are all kinds of volume boost pedals you can get
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If you are recording, record your solo on it's own track and EQ the treble up usually.

If playing live, I use a phaser for a small treble boost. Several metal guitarists do this actually.

If you're looking to make your solo cut through the mix, a much better idea would be to boost the mids. Any eq pedal would enable you to do this. Also most OD's based on the tubescreamer have a fair mid-hump and can be used as a boost rather than an OD.
I use a TubeScreamer, if additional distortion is suitable, there is no better way to boost your sound

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Play on the neck or middle position and switch to the bridge pickup for solos? It's what Buddy Holly (And countless others, too, I'm sure) did.
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depends what you're looking for, a (good) boost pedal will give you a flat volume boost, if you want it eq'd a eq pedal would do. depending on what your're playing oyu could play up to the solo on a lower volume and then simply turn up your guitar's volume
EQ's are the best. You can boost the level and have a flat EQ, if that's what you want, or you can adjust the EQ to give you a different tone, or do both. They're extremely versatile/useful pedals.
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it depends on the type of gear you're using.

if you're using a tube amp the obvious choice would be an overdrive pedal with a bit of a mid-boost, like a tubescreamer, but if you're using a solid state amp i'd recommend something like a compressor instead.
I like analogue Solid State amps that make no effort to be "tube-like", and I'm proud of it...

...A little too proud, to be honest.
Compression or EQ. Overdrive pedal maybe.

Or all three!

I would say that an EQ pedal is your best bet, though. Compression will help a bit, and overdrive too, but I feel overdrive helps more in its mid boost which an EQ pedal would also do.
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I prob would tend to use an overdrive/distortion pedal.

Aside from that is getting an amp with multiple channels.
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Mid boost, output boost before the amp and/or compression are all standard ways to make a hard rock or metal solo stand out more.

For blues and classic rock solos it's more normal to go with a straight output boost before the amp and a volume boost in the power amp section, many classic amp designs have such features built in.

For really old school blues-rock solos the standard thing was to use your guitar's volume and tone controls more. It works well for metal too. Get used to keeping the guitar's volume controls on 4 or 5 and turn up for solos. Gibson style guitars have four controls specifically so you can have one pickup set lower with more treble rolled off on the tone control and then just switch to the other pickup with the controls full on for solos.

Out of phase tones can help a solo be more distinct but you'll want to match that with a significant volume boost.