I work with silicone on a daily basis. The painters tape is a sure fire way to get straight lines but there is an even better way to apply silicon.
Get a spray bottle. Fill it about 3/4 with water, then add dish soap to it. I use dawn, but any should work. You want to put in enough dish soap to turn the water the color of the soap, blue in the case of dawn.
With your spray bottle close by, run your silicon bead into the seam, putting a thick bead and making sure there are no gaps where the silicon isn't touching the glass.
Once you have your silicon bead in place, saturate the glass with the soap spray. Anything the soap touches, silicon won't stick to, including your fingers. Run your finger along the bead (I start at the bottom and work up) wiping the excess silicone off of your finger onto a paper towel as you go.
Use your thumb for a wider bread, or one of your fingers for tighter beads. The good thing about the soap spray is, once you get done, if you notice a rough looking spot you can spray some on your finger and smooth it out really quick without pulling the silicone out of the joint because the silicone won't stick to the spray on your finger.
Using this method, tape is not even needed, but can be used for good measure.
I did this without tape using the above method.
You can see some of the soap spray residue in the picture (the wet area around the silicone). Just wait for the silicone to dry and wash the tank out with water to remove any soap residue.
Takes a little practice, but the results don't lie.