While EQ and compression are the main tools for achieving fatter mixes, reverb and echo are used on nearly all music mixes to bring the song to life and add gloss to the sound.
A good starting point for any mix in Reason is to add two RV7000 reverbs to aux 1 and 2 of your mixer, using the first for reverb and the second for delays. A medium-length plate reverb works well on many sources. Effects connected to an aux send are shared by all mixer channels, so you can add varying amounts of the same reverb and delay to many instruments. Not only is this an efficient use of your CPU power, it helps tie all your sounds together into the same perceptual space and create a sense of wholeness in the mix.
Too much reverb can wash out your mix and detract from its punchiness. One way to avoid this is to use the gate section of the RV7000. This reduces the length of the reverb tail, rather than forcing you to make the reverb too subtle with the decay knob.
Delays always add interest to a mix, and help tie it together rhythmically. However, avoid using echoes on fast rhythmic sounds and loud low-frequency sounds (for example, kick drums). This just creates chaos. If you’re working in Reason 3, try swapping out your RV7000 or DDL1 delay with one of the new Combinator delay-based effect patches.
Remember that you can adjust the level of an effect across all channels at once by tweaking the return master pots at the top right of the mixer.