Colosseum Tickets - what the secret to getting them?

in all honesty, I think your best bet is to book a tour. Part of the tour fee is getting tickets and being able to skip the line. More expensive, obviously, but you get in. That's what we decided to do for our trip that is coming up soon (during high tourist season, but that's when we could go)

What dates are you looking for? I just looked at the website, and only tickets in 30 days (May 27) are available, but about 10 tickets for the Colosseum, Forum and Palantine Hill on May 27th are available at the moment. Tickets for the underground and arena are difficult to come by and as @redheadedfluteplayer suggested, booking a tour will be your best bet for that.

Get an overpriced tour if you want to see the underground level, but don't believe that you can actually skip the security line...

hey hey thomcolon
when is this trip, how many people? tickets for the colosseum are a wanted ticket to get, so many questions on this forum are the same as or like yours. like others here mention maybe a tour if it is that important to you and pay the cost knowing you have the ticket. not worth stressing out to be disappointed. keep an eye out or an alert when they become available as sailbeachbum mentions.
having a ticket you will skip the line but you will still have to go thru a security line, who knows how long the lines will be and wait time. with summer times being so so crowded and HOT. a forum member posted yesterday that he bought viator semi private group tour which put him in a shorter line for entrance "security". both vatican & colosseum tour were worth the investment with no regrets.
many other reseller offers:
getyourguide.com
airbnb.com/experiences/rome
crowntours.com
walksofitaly.com
depending what's important for you to see and do is up to you, prices going up, with big cities in italy being #1 or #2 places to visit, so many thousands & thousands & thousands people traveling. my own opinion knowing all this i would make a "go-go travel account" with direct deposits saving money until trip/vacation came. good luck and enjoy
aloha

No secret. The Colosseum is THE most-visited attraction in Italy. Everyone and their brother wants to get into the thing. 24-hr. standard-entry tickets for the Colosseum, Roman Forum and Palatine usually have the most availability via the park's ticketing website:

https://colosseo.it/en/tickets/24h-colosseum-roman-forum-and-palatine/

Tickets that include the underground are the least available: these critters sell out within minutes of becoming available. Die-hards stay up through the wee hours, and some use multiple computers to try and land them, often to no avail. It just is what it is unless booking a tour through an independent party (avoid booking thru 3rd parties like Viator, Get your Guide, etc.). Personally, having been to the Colosseum twice in my life without seeing the underground, I've never understood the obsession with it: you can see down into a good part of it from above.

Part of the tour fee is getting tickets and being able to skip the
line.

The whole "skip the line" is too often presented as some sort of extra when indeed ALL entries to the Colosseum - tours or individual tickets bought thru the official website or independent tour company - are timed-entry. You skip a ticket line: you do not skip the security-check line. No one gets around that.

So there ya go. I will give you one more option if willing to haul it out of bed early....
There are two ticket kiosks onsite. They will be VERY busy but it's one way to try and get hands on the things. This method did work for a poster last month: here's the thread:

https://community.ricksteves.com/travel-forum/italy/colosseum-ticketing

Click on "Ticket Offices" in the link I'd provided in that thread for location. Note that if you are traveling with others, ALL of you have to be present with legal IDs in hand: your assigned entry time could be sooner rather than not. I'm pretty sure you cannot buy underground tickets at these offices, just standard entry, but won't swear to it. And DO be there WELL before the offices open: Rome will be busier this summer than when the poster in that linked thread was there in March.

Additional caveat: Even if so determined to see that underground that you book a pricey tour through a reputable indy company, be aware that even THOSE tours can go awry. A disclaimer from one of those companies:

"Can my tour be canceled or moved to a different day?
Yes, it is possible. Colosseum Underground tours are very exclusive and are subject to changes, but we’re experts in this field and have staff dedicated to ensuring the best possible customer experience. If we are unable to acquire tickets on your behalf, we will let you know at least 14 days before your tour date, in which case we'll add you to a comparable tour or offer a full refund."

So you see, even the tour companies cannot always get hands on those elusive underground tickets! Some of them have thrown in the towel and stopped selling tours which include that section altogether.