When you have great content and you are helping a lot of people with your blog and its contents its okay to request for a Tip or donation to support your blog.
I usually have different sources of income on the blog be it Adsense, paid ad space, affiliate income but I do not use all of them at a time – it could annoy your blog visitors as your blog could look clogged or spammy. But asking for a donation is not the best method. Ask it in style maybe like “buy me a beer” or “get me a coffee”.
There are many methods to get this implemented and there are few good plugins that could easily be used on your wordpress blog. Just install them on the blog and simply activate the plugin. Configure the settings and you are live. There are different variants of this plugin:
Various Plugin download sources