Years ago these things were made by hand, and be carrying a lot of time, but thanks to some designers that offer large amounts of material ready to use we can fix this in minutes.
Today I want to recommend this excellent resource to have in mind when promoting a party, a concert or meeting. This is an A4 sized poster made entirely in PhotoShop, allowing editing with a simple click and subsequent printing in excellent quality.

Download Club Party Poster | Dezignus
PSD file format is made in layers labeled, you can change the style only changing the color of the forms that compose it, and obviously the possible variations are endless. Figures has really nice bokeh, but nothing removes add their own.
To edit this document absolutely need a particular font called Rocko Ultra FLF which unfortunately could not find for download, but a thinner version, free Fontex . Also this can also be modified as they wish, can use similar fonts (in my case use the Arial Rounded Bold MT), exchanging forms and colors, plus obviously your own text to include promotion of the event.
It is an excellent resource that I found very well designed and simple to change, taking into account the size at the time of export, if they will not print and only need to send by email, should reduce the size to a normal weight according to , this is always better to do at the time of export.