We have several really good methods for advertisement. First and foremost is the Roleplay Ad Banner system. Submitting via this tool will get your RP an ad in our cycle of banners at the top of the forum. If you do not have the graphics skills to make a banner, you can always request one in the Media Shops and Requests forum!
You can also make an advertisement thread in Group Roleplay Advertising. Make sure to use the correct prefix and to provide the basic idea of your RP as well as links to any In-Character and Out-of-Character threads if you want to be successful at snaring players! There are subforums for Redstar/Bluestar ads as well.
If you want to test a premise out and see how much attention it will attract before putting in the effort of planning a roleplay, you can start an Interest Check in Interest Checking & Creation as well.
Checking out the Seeking Group Invites forum is another good way to find players actively searching for RPs. (Again, there are respective Star subforums here.) Pay close attention to whether their tastes match the RP that you're promoting. You can also go to their profile and read their Roleplayer's Resume for more details. Remember that if a member has not posted here or is not someone you know well, do NOT contact them about your RPs. This is spam. Feel free to post your own invitation availability in this forum, too!
For those of you trying to find partners for one-on-ones, the Seeking One on One Partners forum is where you should post your Partner Request thread. (Or, for a sexually-themed RP, your age group's subforum.) Keep in mind, too, that partners for one-on-ones are much more likely to want to plot an RP with you! If you're having trouble finding a partner for a one-on-one thread that you started alone, try instead finding a partner first and floating ideas past them. You can always shelve that plot for later when you find a partner whose interests match up, and in the meantime you can put that energy to better use with the partner you have!
If you're really itching to use a particular character, but don't have a partner for them, we have the Character Gallery. You can post your character in the appropriate public album respective to availability and interest, or create your own personal albums.
As a last resort, you can jump into Iwaku's Discord and check interest with the chatters. However, linking repeatedly or bugging the chatters DOES count as ad spam. Wait until several new chatters come in to link again.
Another thing that can really make all the difference is having an attractive signup or request thread. You can read this Lesson for tips. Make sure, too, that your thread titles have no typos, misspellings, or incorrect grammar! Your thread title is the initial hook for players, and some will dismiss your thread for mistakes in the title before they ever see your awesome plot.
Above all, be patient. It can be very frustrating not to get a response, but losing your cool will only alienate potential players/partners. Relax, have fun, and if a plot doesn't work out, shelve it for later and try something else!